vmd(8): support variable length vionet rx descriptor chains

vmd(8): support variable length vionet rx descriptor chains

The original implementation of the virtio network device assumed a
driver would only provide a 2-descriptor chain for receiving packets.
The virtio spec allows for variable length chains and drivers, in
practice, construct them when they use a sufficiently large MTU.

This change lets the device use variable length chains provided by
the driver, thus allowing for drivers to set an MTU up to the
underlying host-side tap(4)'s limit of TUNMRU (16384).

Size limitations are now enforced on both tx and rx-side dropping
anything violating the underlying tap(4) min and max limits.

More work is needed to increase the read(2) buffer in use by vmd
to prevent packet truncation.

OK mlarkin@