tests/libqos: Replace clock_step with qtest_clock_step in virtio code

Library functions should not rely on functions that require global_qtest
(since they might get used in tests that deal with multiple states).
Commit 1999a70a ("Make generic virtio code independent from
global_qtest") already tried to clean the libqos virtio code, but I
missed to replace the clock_step() function. Thus change it now to
qtest_clock_step() instead.
The logic of the qvirtio_wait_config_isr() function is now pushed
to the virtio-mmio.c and virtio-pci.c files instead, since we can
get the QTestState here easily.

Message-Id: <20190904130047.25808-4-thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>