virfile: Rename virFileUnlink to virFileRemove

virfile: Rename virFileUnlink to virFileRemove

Similar to commit id '35847860', it's possible to attempt to create
a 'netfs' directory in an NFS root-squash environment which will cause
the 'vol-delete' command to fail.  It's also possible error paths from
the 'vol-create' would result in an error to remove a created directory
if the permissions were incorrect (and disallowed root access).

Thus rename the virFileUnlink to be virFileRemove to match the C API
functionality, adjust the code to following using rmdir or unlink
depending on the path type, and then use/call it for the VIR_STORAGE_VOL_DIR