Monitor: Fix command execution regression

Function is_async_return() added by commit 940cc30d0d4 assumes
that 'data', which is returned by handlers, is always a QDict.

This is not true, as QLists can also be returned, in this case
we'll get a segfault.

Fix that by checking if 'data' is a QDict.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
