hyperv: set receive buffer size according to NVSP protocol version

If the NVSP protocol version is not greater than NVSP_PROTOCOL_VERSION_2,
then the recv buffer size is 15MB, otherwise the buffer size is 16MB.

Submitted by: Hongjiang Zhang <honzhan microsoft com>
Reviewed by: royger, Dexuan Cui <decui microsoft com>, adrian
Approved by: adrian (mentor)
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D4814