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>