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hyperv/hn: Factor out hn_nvs_send/hn_nvs_send_sglist

Description

hyperv/hn: Factor out hn_nvs_send/hn_nvs_send_sglist

Avoid unnecessary message type setting and centralize the send context
to transaction id cast.

MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D7500