MFC @ r259205 in preparation for some SVM updates. (for real this time)

Merge head r233826 through r256722.

Merge head@256284

MFC - tracking commit

Fix vmbus channel memory leak where incorrect length parameter was
being passed to contigfree().

Submitted by: Microsoft hyperv dev team
Approved by: re@ (glebius)

IFC @256277

Approved by: ken (mentor)

MFC - tracking update.

IFC @ r255692

Comment out IA32_MISC_ENABLE MSR access - this doesn't exist on AMD.
Need to sort out how arch-specific MSRs will be handled.

Import Hyper-V paravirtualized drivers from projects/hyperv
branch into head.

Approved by: re@ (hrs)
Obtained from: Microsoft, NetApp, and Citrix.

Microsoft have changed their policy on how the hyper-v code will
be pulled into FreeBSD. From now, FreeBSD will be considered the
upstream repo.

First step: move the drivers away from the contrib ar

Microsoft have changed their policy on how the hyper-v code will
be pulled into FreeBSD. From now, FreeBSD will be considered the
upstream repo.

First step: move the drivers away from the contrib area and into
the base system.

A follow-on commit will include the drivers in the amd64 GENERIC kernel.

show more ...

Initial import of the Microsoft HyperV 'enlightened' drivers.

From https://github.com/FreeBSDonHyper-V/VendorBranchForFreeBSDonHyper-V
rev: 99eaa0ddb0485c9d76046664100f6beb1a0a0c58