tests: qemuMonitorTestProcessCommandDefaultValidate: simplify condition
tests: qemuMonitorTestProcessCommandDefaultValidate: simplify condition

We return success when running this function for either non-JSON monitor
testing or guest agent testing.

However we no longer test HMP monitor and we do not try to validate
the guest agent interaction.

Drop the test->json check and report a proper error if someone tries
to run this function for the guest agent without properly wiring it up.

Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

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