virtio: Implement notify_status

Modern virtio require proper status handling and reset. A "notify_status"
callback is already present in the virtio ops, but isn't implemented by
any device. Instead they currently use "set_guest_feature" to reset the
device and deal with endianess. This isn't sufficient for proper device
reset, so add the notify_status callback to all devices that need it.

To add useful hints like "start" and "stop", extend the status variable to
32-bits.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
[Julien T: Remove VIRTIO_CONFIG_S_NEEDS_RESET from config mask, as
	it is virtio v1+ macro and kvmtool only implements v0.9, this
	macro should not be referenced for now]
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>