virtiofs: forbid newlines in tags

[ Upstream commit 40488cc16f7ea0d193a4e248f0d809c25cc377db ]

Newlines in virtiofs tags are awkward for users and potential vectors
for string injection attacks.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>