qapi/common: Fix guardname() for funny filenames

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 <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <20180211093607[email protected]>
Reviewed-by: Michael Roth <[email protected]>
Signed-off-by: Eric Blake <[email protected]>