qapi: protect against NULL QObject in qmp_input_get_object

A NULL qobj can occur when a parameter is fetched via qdict_get, but
the parameter is not in the command.  By returning NULL, the caller can
choose whether to raise a missing parameter error, an invalid parameter
type error, or use a default value.  For example, qom-set could can
use this to reset a property to its default value, though at this time
it will fail with "Invalid parameter type".  In any case, anything is
better than crashing!

Reviewed-by: Anthony Liguori <anthony@codemonkey.ws>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
