qemu: Label correct per-VM path when starting
    
    Commit f1f68ca33433825ce0deed2d96f1990200bc6618 overused mdir_name()
    event though it was not needed in the latest version, hence labelling
    directory one level up in the tree and not the one it should.
    
    If anyone with SElinux managed to try run a domain with guest agent set
    up, it's highly possible that they will need to run 'restorecon -F
    /var/lib/libvirt/qemu/channel/target' to fix what was done.
    
    Reported-by: Luyao Huang <lhuang@redhat.com>
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>