tests/qtest: Add a function to check whether a machine is available

It is nowadays possible to build QEMU with a reduced set of machines
in each binary. However, the qtests still hard-code the expected
machines and fail if the binary does not feature the required machine.
Let's get a little bit more flexible here: Add a function that can be
used to query whether a certain machine is available or not, and use
it in some tests as an example (more work has to be done in other
tests which will follow later).

Message-Id: <20211201104347.51922-5-thuth@redhat.com>
Acked-by: John Snow <jsnow@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/qtest/libqos/libqtest.h

@@ -710,6 +710,14 @@ QDict *qmp_fd(int fd, const char *fmt, ...) GCC_FMT_ATTR(2, 3);
 void qtest_cb_for_every_machine(void (*cb)(const char *machine),
                                 bool skip_old_versioned);
 
+/**
+ * qtest_has_machine:
+ * @machine: The machine to look for
+ *
+ * Returns: true if the machine is available in the target binary.
+ */
+bool qtest_has_machine(const char *machine);
+
 /**
  * qtest_qmp_device_add_qdict:
  * @qts: QTestState instance to operate on