tests/qtest: remove clock_steps from virtio tests

tests/qtest: remove clock_steps from virtio tests

In the qtest environment time will not step forward if the system is
paused (timers disabled) or we have no timer events to fire. As a
result VirtIO events are responded to directly and we don't need to
step time forward.

We still do timeout processing to handle the fact the target QEMU may
not be ready to respond right away. This will usually be due to a slow
CI system or if QEMU is running under something like rr.

Future qtest patches will assert that time actually changes when a
step is requested.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250108121054.1126164-17-alex.bennee@linaro.org>