virfile: Introduce virFileUnlink

virfile: Introduce virFileUnlink

In an NFS root-squashed environment the 'vol-delete' command will fail to
'unlink' the target volume since it was created under a different uid:gid.

This code continues the concepts introduced in virFileOpenForked and
virDirCreate[NoFork] with respect to running the unlink command under
the uid/gid of the child. Unlike the other two, don't retry on EACCES
(that's why we're here doing this now).