qapi: fix memory leak on error

QmpInputVisitor would leak the malloced struct if the stack was
overflowed.  This can be easily fixed using error_propagate.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
