include/hw: VM state takes precedence in virtio_device_should_start

include/hw: VM state takes precedence in virtio_device_should_start

The VM status should always preempt the device status for these
checks. This ensures the device is in the correct state when we
suspend the VM prior to migrations. This restores the checks to the
order they where in before the refactoring moved things around.

While we are at it lets improve our documentation of the various
fields involved and document the two functions.

Fixes: 9f6bcfd99f (hw/virtio: move vm_running check to virtio_device_started)
Fixes: 259d69c00b (hw/virtio: introduce virtio_device_should_start)
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20221130112439.2527228-6-alex.bennee@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>