hyperv/vmbus: make hyperv eventtimer as a module

[howard0su_gmail.com]

Summary

Also simplify eventtimer to only support one-shot.

Test Plan

Run the following command to switch hyperv eventtimer:
sysctl kern.eventtimer.timer=HyperV

also run devinfo -v to make sure hv_et is there and under
vmbus

[decui_microsoft.com]

sys/dev/hyperv/vmbus/hv_vmbus_drv_freebsd.c

292

Must we check this? It looks almost all the callers of device_get_ivars(child) don't have the check.
If it's a must, we should fix the other callers of device_get_ivars() too, e.g., vmbus_read_ivar(), vmbus_write_ivar().

573

what's the meaning of "delay attach until inerrupt is on" -- I don't see how bus_generic_attach() can delay things?

At this place, the vmbus doesn't have any child device yet?

[sepherosa_gmail.com]

sys/dev/hyperv/vmbus/hv_vmbus_drv_freebsd.c

573

He tries to attach the hv_et here. Rest of the devices go through channel offers later.

[decui_microsoft.com]

sys/dev/hyperv/vmbus/hv_vmbus_drv_freebsd.c

573

Got it. Thanks!