For the upcoming patches, we will need a way to gets a list with all
available machine types. Refactor the qtest_cb_for_every_machine()
to split the related code out into a separate new function, and
gather the aliases of the various machine types, too.

Message-Id: <20211201104347.51922-4-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>