qapi: Check for negative enum values

We don't currently check for negative enum values in qmp_output_type_enum(),
this will very likely generate a segfault when triggered.

However, it _seems_ that no code in tree can trigger this today.

Acked-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
