uuid: Tighten uuid parse

sscanf is relatively loose (tolerate) on some invalid formats that we
should fail instead of generating a wrong uuid structure, like with
whitespaces and short strings.

Add and use a helper function to first check the format.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-Id: <1474432046-325-11-git-send-email-famz@redhat.com>
