Move xen interrupt handlers to dedicated task queues

Move xen interrupt handlers to dedicated task queues

Handling receive and transmit for multiple networking interfaces
in a "shared interrupt" within normal interrupt vector code path
introduces too much delay from the hypervisor POV which prevents
it from injecting further completion event interrupts for Rx and
Tx queues.

Additionally, Netfront backend driver includes a mechanism to
detect Rx ring stalls and "turn the carrier off" when the guest
is not replenishing the ring (e.g. due to missing completion
interrupts) that relies on guest waking up periodically and making
sure that the Rx ring completion handling is progressing.

Having tried both task queue + timeout and interrupts + timeout
approaches, it appears that using  the task queue is more flexible
and provides superior performance under heavy network load.