vtnet: fix panic on unload

Since r276367 added the virtio_mmio support vtnet_modevent() gets called twice.
This resulted in a memory leak during load and a panic on unload.

Count the loads so we only initialise once (just like cxgbe(4)), and only clean
up in the final unload.

PR: 209428
Submitted by: novel@FreeBSD.org
MFC after: 1 week