tests/qtest/migration: Allow user to specify a machine type

tests/qtest/migration: Allow user to specify a machine type

Accept the QTEST_QEMU_MACHINE_TYPE environment variable to take a
machine type to use in the tests.

The full machine type is recognized (e.g. pc-q35-8.2). Aliases
(e.g. pc) are also allowed and resolve to the latest machine version
for that alias, or, if using two QEMU binaries, to the latest common
machine version between the two.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231018192741.25885-12-farosas@suse.de>