tests/qtest: Allow qtest_get_machines to use an alternate QEMU binary

tests/qtest: Allow qtest_get_machines to use an alternate QEMU binary

We're adding support for using more than one QEMU binary in
tests. Modify qtest_get_machines() to take an environment variable
that contains the QEMU binary path.

Since the function keeps a cache of the machines list in the form of a
static variable, refresh it any time the environment variable changes.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231018192741.25885-4-farosas@suse.de>