qapi: Use an explicit input file

qapi: Use an explicit input file

Use an explicit input file on the command-line instead of reading from standard
input.

It also outputs the proper file name when there's an error.

Signed-off-by: Lluís Vilanova <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Luiz Capitulino <[email protected]>