MFC r256425:

Centralize the detection logic for the Hyper-V hypervisor.

Submitted by:	Roger Pau Monné
Sponsored by:	Citrix Systems R&D
Reviewed by:	gibbs, grehan
Approved by:	re (gjb)

sys/sys/systm.h:
 * Add a new VM_GUEST type, VM_GUEST_HV (HyperV guest).

sys/dev/hyperv/vmbus/hv_vmbus_drv_freebsd.c:
sys/dev/hyperv/vmbus/hv_hv.c:
sys/dev/hyperv/stordisengage/hv_ata_pci_disengage.c:
 * Set vm_guest to VM_GUEST_HV and use that on other HyperV related
   devices instead of cloning the cpuid hypervisor check.
 * Cleanup the vmbus_identify function.
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