qemuDomainRemoveMemoryDevice: unlink() memory backing file

qemuDomainRemoveMemoryDevice: unlink() memory backing file

https://bugzilla.redhat.com/show_bug.cgi?id=1461214

Since fec8f9c49af we try to use predictable file names for
'memory-backend-file' objects. But that made us provide full path
to qemu when hot plugging the object while previously we provided
merely a directory. But this makes qemu behave differently. If
qemu sees a path terminated with a directory it calls mkstemp()
and unlinks the file immediately. But if it sees full path it
just calls open(path, O_CREAT ..); and never unlinks the file.
Therefore it's up to libvirt to unlink the file and not leave it
behind.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>