Introduce a per-VNET flag to enable/disable netisr prcessing on that VNET.
Add accessor functions to toggle the state per VNET.
The base system (vnet0) will always enable itself with the normal
registration. We will share the registered protocol handlers in all
VNETs minimising duplication and management.
Update netisr consumers to (de)register on a per-VNET start/teardown using
VNET_SYS(UN)INIT functionality.
Upon disabling netisr processing for a VNET drain the netisr queue.
The change should be transparent for non-VIMAGE kernels.