if_vtnet: Add support for CTRL_GUEST_OFFLOADS feature

This allows the Rx checksum and LRO to be modified without a full
reinit of the device.

Remove IFCAP_RXCSUM_IPV6 from the interface capabilities since in
VirtIO Rx checksums are just enabled or disabled for all protocols.

Properly update IFCAP_LRO if LRO is becomes disabled when Rx
checksums are disabled.

Reviewed by: grehan (mentor)
Differential Revision: https://reviews.freebsd.org/D27916