i440fx-test: give each GTest case its own qtest

i440fx-test: give each GTest case its own qtest

The current two GTest cases, /i440fx/defaults and /i440fx/pam can share a
qemu process, but the next two cases will need dedicated instances. It is
messy (and order-dependent) to dynamically configure GTest cases one by
one to start, stop, or keep the current qtest (*); let's just have each
GTest work with its own qtest. The performance difference should be
negligible.

(*) As g_test_run() can be invoked at most once per process startup, and
it runs GTest cases in sequence, we'd need clumsy data structures to
control each GTest case to start/stop/keep the qemu instance. Or, we'd
have to code the same information into the test methods themselves, which
would make them even more order-dependent.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>