macvtap: log an error if on failure to connect to netlink socket
macvtap: log an error if on failure to connect to netlink socket

A bug in libnl (see https://bugzilla.redhat.com/show_bug.cgi?id=677724
and https://bugzilla.redhat.com/show_bug.cgi?id=677725) makes it very
easy to create a failure to connect to the netlink socket when trying
to open a macvtap network device ("type='direct'" in domain interface
XML). When that error occurred (during a call to libnl's nl_connect()
from libvirt's nlComm(), there was no log message, leading virsh (for
example) to report "unknown error".

There were two other cases in nlComm where an error in a libnl
function might return with failure but no error reported. In all three
cases, this patch logs a message which will hopefully be more useful.

Note that more detailed information about the failure might be
available from libnl's nl_geterror() function, but it calls
strerror(), which is not threadsafe, so we can't use it.

bjzhang/libvirt,bjzhang/libvirt,bjzhang/libvirt,agx/libvirt,emaste/libvirt,foomango/libvirt,dumbbell/libvirt,cbosdo/libvirt,nertpinx/libvirt,datto/libvirt,bjzhang/libvirt,nertpinx/libvirt,zhlcindy/libvirt-1.1.4-maintain,dumbbell/libvirt,foomango/libvirt,emaste/libvirt,shugaoye/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,eskultety/libvirt,VenkatDatta/libvirt,eskultety/libvirt,foomango/libvirt,rbu/libvirt,emaste/libvirt,novel/fbsd-libvirt,soulxu/libvirt-xuhj,rbu/libvirt,libvirt/libvirt,libvirt/libvirt,shugaoye/libvirt,VenkatDatta/libvirt,zippy2/libvirt,wiedi/libvirt,elmarco/libvirt,danwent/libvirt-ovs,jfehlig/libvirt,andreabolognani/libvirt,emaste/libvirt,wiedi/libvirt,emaste/libvirt,siboulet/libvirt-openvz,fabianfreyer/libvirt,kantai/libvirt-vfork,crobinso/libvirt,warewolf/libvirt,novel/fbsd-libvirt,eskultety/libvirt,cbosdo/libvirt,leilihh/libvirt,novel/fbsd-libvirt,iam-TJ/libvirt,jardasgit/libvirt,jeckersb/libvirt,novel/fbsd-libvirt,dumbbell/libvirt,taget/libvirt,taget/libvirt,soulxu/libvirt-xuhj,leilihh/libvirt,olafhering/libvirt,usc-isi/libvirt,emaste/libvirt,novel/fbsd-libvirt,trainstack/libvirt,wiedi/libvirt,rmarwaha/libvirt,foomango/libvirt,kantai/libvirt-vfork,datto/libvirt,leilihh/libvirt,rmarwaha/libvirt,trainstack/libvirt,taget/libvirt,foomango/libvirt,emaste/libvirt,nertpinx/libvirt,kantai/libvirt-vfork,rmarwaha/libvirt1,eskultety/libvirt,novel/fbsd-libvirt,warewolf/libvirt,rbu/libvirt,trainstack/libvirt,shugaoye/libvirt,rbu/libvirt,iam-TJ/libvirt,jeckersb/libvirt,cbosdo/libvirt,bjzhang/libvirt,jardasgit/libvirt,zippy2/libvirt,wiedi/libvirt,andreabolognani/libvirt,rlaager/libvirt,trainstack/libvirt,elmarco/libvirt,agx/libvirt,iam-TJ/libvirt,rlaager/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,agx/libvirt,zhlcindy/libvirt-1.1.4-maintain,elmarco/libvirt,danwent/libvirt-ovs,wiedi/libvirt,wiedi/libvirt,rmarwaha/libvirt,siboulet/libvirt-openvz,rlaager/libvirt,rmarwaha/libvirt1,usc-isi/libvirt,cbosdo/libvirt,danwent/libvirt-ovs,trainstack/libvirt,VenkatDatta/libvirt,soulxu/libvirt-xuhj,rmarwaha/libvirt1,jfehlig/libvirt,kantai/libvirt-vfork,warewolf/libvirt,jeckersb/libvirt,taget/libvirt,crobinso/libvirt,zippy2/libvirt,leilihh/libvirt,rmarwaha/libvirt1,soulxu/libvirt-xuhj,soulxu/libvirt-xuhj,danwent/libvirt-ovs,eskultety/libvirt,siboulet/libvirt-openvz,rbu/libvirt,jeckersb/libvirt,fabianfreyer/libvirt,rmarwaha/libvirt1,andreabolognani/libvirt,wiedi/libvirt,dumbbell/libvirt,novel/fbsd-libvirt,jeckersb/libvirt,leilihh/libvirt,warewolf/libvirt,zippy2/libvirt,usc-isi/libvirt,warewolf/libvirt,trainstack/libvirt,usc-isi/libvirt,iam-TJ/libvirt,elmarco/libvirt,warewolf/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,jfehlig/libvirt,danwent/libvirt-ovs,crobinso/libvirt,nertpinx/libvirt,jeckersb/libvirt,iam-TJ/libvirt,dumbbell/libvirt,rmarwaha/libvirt,datto/libvirt,rmarwaha/libvirt,libvirt/libvirt,rlaager/libvirt,novel/fbsd-libvirt,jeckersb/libvirt,novel/fbsd-libvirt,olafhering/libvirt,datto/libvirt,elmarco/libvirt,leilihh/libvirt,fabianfreyer/libvirt,kantai/libvirt-vfork,olafhering/libvirt,taget/libvirt,cbosdo/libvirt,trainstack/libvirt,iam-TJ/libvirt,crobinso/libvirt,zhlcindy/libvirt-1.1.4-maintain,siboulet/libvirt-openvz,agx/libvirt,rlaager/libvirt,agx/libvirt,usc-isi/libvirt,iam-TJ/libvirt,rmarwaha/libvirt,zhlcindy/libvirt-1.1.4-maintain,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,olafhering/libvirt,jfehlig/libvirt,shugaoye/libvirt,siboulet/libvirt-openvz,jardasgit/libvirt,jardasgit/libvirt,shugaoye/libvirt,warewolf/libvirt,libvirt/libvirt,rmarwaha/libvirt1,datto/libvirt,dumbbell/libvirt