if_vtnet: Schedule Rx task if pending items when enabling interrupt

Prior to V1, the driver would enable interrupts and then notify the
host that DRIVER_OK. Since for V1, DRIVER_OK needs to be set before
notifying the virtqueues, there may be items in the queues waiting
to be processed by the time interrupts are enabled.

This fixes a bug where the Rx queue would appear stuck, only being
usable after an interface down/up cycle.

Reviewed by: grehan (mentor)
Differential Revision: https://reviews.freebsd.org/D27922