qemu: process: Report better error when virtlogd connection fails

qemu: process: Report better error when virtlogd connection fails

When connecting to virtlogd fails e.g. due to wrong libvirtd selinux
process label we'd report an utterly useless error message:

$ virsh start upstream
error: Failed to start domain upstream
error: Cannot recv data: Connection reset by peer

Use virLastErrorPrefixMessage in the correct place to give a better
sense of what's going on:

$ virsh start upstream
error: Failed to start domain upstream
error: can't connect to virtlogd: Cannot recv data: Connection reset by peer

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>