hyperv: vmbus: refactor msg handler by replacing swi with task queue

[honzhan_microsoft.com]

Summary

swi launches ithread to handle msg from host, which is not efficient
enough. Processing the message directly in task queue during software
interrupt is preferred.

Submitted by: Hongjiang Zhang <honzhan microsoft com>

[howard0su_gmail.com]

sys/dev/hyperv/vmbus/hv_vmbus_drv_freebsd.c

539

no need create dedicate task queue. you can use taskqueue_fast directly.

[honzhan_microsoft.com]

There is another succinct implementation with taskqueue_fast. Please reference https://reviews.freebsd.org/D5215

[howard0su_gmail.com]

a different version with same idea is already committed.