tests: use qtest_pc_boot()/qtest_shutdown() in virtio tests

This patch replaces calls to qtest_start() and qtest_end() by
calls to qtest_pc_boot() and qtest_shutdown().

This allows to initialize memory allocator and PCI interface
functions. This will ease to enable virtio tests on other
architectures by only adding a specific qtest_XXX_boot() (like
qtest_spapr_boot()).

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
