qapi/common: Fix guardname() for funny filenames

guardname() fails to return a valid C identifier for arguments
containing anything but [A-Za-z0-9_.-'].  Fix that.  Don't bother
protecting ticklish identifiers; header guards are all-caps, and no
ticklish identifiers are.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180211093607.27351-22-armbru@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>