tests/qtest/qos-test: dump QEMU command if verbose

If qtests are run in verbose mode (i.e. if --verbose CL argument
was provided) then print the assembled qemu command line for each
test.

Use qos_printf() instead of g_test_message() to avoid the latter
cluttering the output.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <110bef3595cb841dfa1b86733c174ac9774eb37e.1611704181.git.qemu_oss@crudebyte.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>