scripts: qmp-shell: Add verbose flag

Add a verbose flag that shows the QMP command that was
constructed, to allow for later copy/pasting, reference,
debugging, etc.

The QMP is converted from a Python literal to JSON first,
to ensure that it is viable input to the actual QMP parser.

As a side-effect, this JSON output will helpfully show all
the necessary conversions that were performed on the input,
illustrating that "True" was transformed back into "true",
literal values are now escaped with "" instead of '', and so on.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>