qemu: raise an eror when using aio=native without cache=none
qemu: raise an eror when using aio=native without cache=none

Qemu will fallback to aio=threads when the cache mode doesn't use
O_DIRECT, even if aio=native was explictly set.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1086704

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

olafhering/libvirt,jfehlig/libvirt,crobinso/libvirt,shugaoye/libvirt,libvirt/libvirt,fabianfreyer/libvirt,rlaager/libvirt,agx/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,libvirt/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,zippy2/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,taget/libvirt,jardasgit/libvirt,rlaager/libvirt,agx/libvirt,nertpinx/libvirt,rlaager/libvirt,agx/libvirt,crobinso/libvirt,rlaager/libvirt,taget/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,shugaoye/libvirt,andreabolognani/libvirt,shugaoye/libvirt,andreabolognani/libvirt,olafhering/libvirt,jfehlig/libvirt,jardasgit/libvirt,zippy2/libvirt,olafhering/libvirt,taget/libvirt,elmarco/libvirt,fabianfreyer/libvirt,libvirt/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,eskultety/libvirt,nertpinx/libvirt,datto/libvirt,jardasgit/libvirt,crobinso/libvirt,datto/libvirt,nertpinx/libvirt,agx/libvirt,zippy2/libvirt,datto/libvirt,olafhering/libvirt,nertpinx/libvirt,crobinso/libvirt,shugaoye/libvirt,agx/libvirt,andreabolognani/libvirt,nertpinx/libvirt,elmarco/libvirt,jardasgit/libvirt,datto/libvirt,jardasgit/libvirt,andreabolognani/libvirt,elmarco/libvirt,VenkatDatta/libvirt,eskultety/libvirt,elmarco/libvirt,rlaager/libvirt,datto/libvirt,fabianfreyer/libvirt,taget/libvirt,eskultety/libvirt,eskultety/libvirt,eskultety/libvirt,taget/libvirt,elmarco/libvirt