KVM: x86: Add support for "protected VMs" that can utilize private memory

Add a new x86 VM type, KVM_X86_SW_PROTECTED_VM, to serve as a development
and testing vehicle for Confidential (CoCo) VMs, and potentially to even
become a "real" product in the distant future, e.g. a la pKVM.

The private memory support in KVM x86 is aimed at AMD's SEV-SNP and
Intel's TDX, but those technologies are extremely complex (understatement),
difficult to debug, don't support running as nested guests, and require
hardware that's isn't universally accessible.  I.e. relying SEV-SNP or TDX
for maintaining guest private memory isn't a realistic option.

At the very least, KVM_X86_SW_PROTECTED_VM will enable a variety of
selftests for guest_memfd and private memory support without requiring
unique hardware.

Signed-off-by: Sean Christopherson <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Fuad Tabba <[email protected]>
Tested-by: Fuad Tabba <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

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