xen-netfront: switch to using an interrupt handler

In order to use custom taskqueues we would have to mask the interrupt, which
is basically what is already done for an interrupt handler, or else we risk
loosing interrupts. This switches netfront to the same interrupt handling
that was done before multiqueue support was added.

Reviewed by: Wei Liu <wei.liu2@citrix.com>
Sponsored by: Citrix Systems R&D