qtest: switch users back to qtest_qmp_receive

Let test use the new functionality for buffering events.
The only remaining users of qtest_qmp_receive_dict are tests
that fuzz the QMP protocol.

Tested with 'make check-qtest'.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20201006123904.610658-4-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>