hyperv: register hv_timecounter in SI_SUB_HYPERVISOR

[decui_microsoft.com]

Summary

hv_timecounter only needs rdmsr() and doesn't depend on the infrastructure
suppied by the whole vmbus driver (i.e., interrupt, channel, etc.), so we
can move it to an earlier place. This way, we can reliably use DELAY() in
probe_tsc_freq() -> DELAY().

Note: the i8254 PIT counter emulated by Hyper-V is not reliable and it even
doesn't exist in a Hyper-V Generation-2 VM (UEFI VM):
https://reviews.freebsd.org/D5174

With this patch, the TSC frequency is properly calibrated and the below
warnings will go away:

calcru: runtime went backwards from 18 usec to 9 usec for pid 0 (kernel)
calcru: runtime went backwards from 46204978 usec to 23362331 usec for pid 0 (kernel)

Test Plan

Boot the kernel with boot_verbose="YES", and check dmesg:

dmesg | grep "TSC clock"
Calibrating TSC clock ... TSC clock: 2500003082 Hz
(This is the correct TSC frequency. Without the patch, the detected TSC
frequency is only ~1.3GHz in my test.)

[kib]

I successfully run with the change for some time.