KVM: selftests: Allow tagging protected memory in guest page tables

Add support for tagging and untagging guest physical address, e.g. to
allow x86's SEV and TDX guests to embed shared vs. private information in
the GPA.  SEV (encryption, a.k.a. C-bit) and TDX (shared, a.k.a. S-bit)
steal bits from the guest's physical address space that is consumed by the
CPU metadata, i.e. effectively aliases the "real" GPA.

Implement generic "tagging" so that the shared vs. private metadata can be
managed by x86 without bleeding too many details into common code.

Cc: Paolo Bonzini <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Vishal Annapurve <[email protected]>
Cc: Ackerly Tng <[email protected]>
cc: Andrew Jones <[email protected]>
Cc: Tom Lendacky <[email protected]>
Cc: Michael Roth <[email protected]>
Tested-by: Carlos Bilbao <[email protected]>
Originally-by: Michael Roth <[email protected]>
Signed-off-by: Peter Gonda <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

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