tests: add QMP input visitor test for unions with no discriminator

tests: add QMP input visitor test for unions with no discriminator

This is more of an exercise of the dealloc visitor, where it may
erroneously use an uninitialized discriminator field as indication
that union fields corresponding to that discriminator field/type are
present, which can lead to attempts to free random chunks of heap
memory.

Cc: qemu-stable@nongnu.org
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
(cherry picked from commit cb55111b4e425fa3279302fa7306b9a3d5164ff4)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>