Checking the value of qmp_dispatch() is repetitive.  Factor out
helpers do_qmp_dispatch() and do_qmp_dispatch_error().  Without this,
the next commit would make things even more repetitive.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200317115459.31821-6-armbru@redhat.com>