hyperv/hn: Rename TX related function and struct fields a bit

Preamble to implement the ifnet.if_transmit method.

Reviewed by:        adrian
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5344

hyperv/hn: Staticize and rename packet TX done function

It is only used in hv_netvsc_drv_freebsd.c; and rename it to hn_tx_done()
mainly to reserve "xmit" for ifnet.if_transmit implement.

While I'm here, remove unapplied comment.

Reviewed by:        adrian
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5345

hyperv/hn: Add TX method for txeof processing.

Preamble to implement ifnet.if_transmit method.

Reviewed by:        adrian
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5346

hyperv/hn: Factor out hn_send_pkt() from hn_start_locked()

It will be shared w/ the upcoming ifnet.if_transmit method
implementation.

No functional change.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5385

hyperv/hn: Use IFQ_DRV_PREPEND instead of IF_PREPEND

IF_PREPEND promises out-of-order packet sending when the TX desc list
is depleted. It was overlooked and copied blindly when the transmission
path was partially rewritten.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5386