The @src is not always a file. It may also be a directory (for
instance qemuDomainCreateDeviceRecursive() assumes that) - even
though it doesn't happen usually. Anyway, mount() can mount only
a dir onto a dir and a file onto a file.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com> Reviewed-by: Pavel Mores <pmores@redhat.com>