virtiofs: include a newline in sysfs tag

The internal tag string doesn't contain a newline. Append one when
emitting the tag via sysfs.

[Stefan] Orthogonal to the newline issue, sysfs_emit(buf, "%s", fs->tag) is
needed to prevent format string injection.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Fixes: a8f62f50 ("virtiofs: export filesystem tags through sysfs")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>