hyperv: Register Hyper-V timer early enough for TSC freq calibration

The i8254 simulation in Hyper-V is kinda broken and is not available
in Generation 2 Hyper-V VMs, so Hyper-V timer must be registered early
enough so that it can be used to do the TSC freq calibration.

This fixes the notorious warning like this:
calcru: runtime went backwards from 50 usec to 25 usec for pid 0 (kernel)

Submitted by:	Dexuan Cui <decui microsoft com>
Reviewed by:	kib, sephe
Tested by:	kib, sephe
MFC after:	1 week
Sponsored by:	Microsoft OSTC
Differential Revision:	https://reviews.freebsd.org/D5778

sys/dev/hyperv/vmbus/hv_hv.c

@@ -36,6 +36,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/malloc.h>
 #include <sys/pcpu.h>
 #include <sys/timetc.h>
+#include <sys/kernel.h>
 #include <machine/bus.h>
 #include <machine/md_var.h>
 #include <vm/vm.h>
@@ -207,8 +208,6 @@ hv_vmbus_init(void)
 
 	hv_vmbus_g_context.hypercall_page = virt_addr;
 
-	tc_init(&hv_timecounter); /* register virtual timecount */
-
 	hv_et_init();
 
 	return (0);
@@ -437,3 +436,14 @@ void hv_vmbus_synic_cleanup(void *arg)
 	wrmsr(HV_X64_MSR_SIEFP, siefp.as_uint64_t);
 }
 
+static void
+hv_tc_init(void)
+{
+	if (vm_guest != VM_GUEST_HV)
+		return;
+
+	/* register virtual timecounter */
+	tc_init(&hv_timecounter);
+}
+
+SYSINIT(hv_tc_init, SI_SUB_HYPERVISOR, SI_ORDER_FIRST, hv_tc_init, NULL);