qemuDomainBuildNamespace: Clean up temp files

qemuDomainBuildNamespace: Clean up temp files

https://bugzilla.redhat.com/show_bug.cgi?id=1431112

After 290a00e41d we know how to deal with file mount points.
However, when cleaning up the temporary location for preserved
mount points we are still calling rmdir(). This won't fly for
files. We need to call unlink(). Now, since we don't really care
if the cleanup succeeded or not (it's the best effort anyway), we
can call both rmdir() and unlink() without need for
differentiation between files and directories.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>