xenapi: Improve error message on session failure
xenapi: Improve error message on session failure

XenAPI session login can fail for a number of reasons, but currently no
specific
reason is displayed to the user, e.g.:

virsh -c XenAPI://citrix-xen.example.com/
Enter username for citrix-xen.example.com: root
Enter root's password for citrix-xen.example.com:
error: authentication failed: (null)
error: failed to connect to the hypervisor

This patch displays the session error description on failure.

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