Add libnvmm, NetBSD's new virtualization API. It provides a way for VMM
software to effortlessly create and manage virtual machines via NVMM.

It is mostly complete, only nvmm_assist_mem needs to be

Add libnvmm, NetBSD's new virtualization API. It provides a way for VMM
software to effortlessly create and manage virtual machines via NVMM.

It is mostly complete, only nvmm_assist_mem needs to be filled -- I have
a draft for that, but it needs some more care. This Mem Assist should
not be needed when emulating a system in x2apic mode, so theoretically
the current form of libnvmm is sufficient to emulate a whole class of
systems.

Generally speaking, there are so many modes in x86 that it is difficult
to handle each corner case without introducing a ton of checks that just
slow down the common-case execution. Currently we check a limited number
of things; we may add more checks in the future if they turn out to be
needed, but that's rather low priority.

Libnvmm is compiled and installed only on amd64. A man page (reviewed by
wiz@) is provided.

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