virtio-ccw: complete handling of guest-initiated resets

For a guest-initiated reset, we need to not only reset the virtio device,
but also reset the VirtioCcwDevice into a clean state. This includes
resetting the indicators, or else a guest will not be able to e.g.
switch from classic interrupts to adapter interrupts.

Split off this routine into a new function virtio_ccw_reset_virtio()
to make the distinction between resetting the virtio-related devices
and the base subchannel device clear.

CC: qemu-stable@nongnu.org
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>