if_vtnet: Add support for software LRO

This useful when running on hosts that support checksum offloading
but not the GUEST_TSO (LRO) feature. Or potentially, some GRO-like
support when doing forwarding.

Only enable SW LRO when the host LRO is not available since both
tends to be harmful, and difficult to enable/disable selectively
with only a single IFCAP_LRO flag.

Reviewed by: grehan (mentor)
Differential Revision: https://reviews.freebsd.org/D27919