tests/pxe: Test more NICs when running in SPEED=slow mode

The pxe-test is a very good test to excercise NICs, thus we should use
it to test all NICs that can be used by the BIOS for booting via network.
However, to avoid that the default testing time increases too much, the
additional NICs are only tested in the "make check SPEED=slow" mode.

The virtio-net NIC on ppc64 is now also only tested in slow mode, since
the test on ppc64 is really quite slow and we've got test coverage for
virtio-net in big endian mode now on s390x, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
