KVM: arm64: Annotate guest donations with handle and gfn in host stage-2

KVM: arm64: Annotate guest donations with handle and gfn in host stage-2

Handling host kernel faults arising from accesses to donated guest
memory will require an rmap-like mechanism to identify the guest mapping
of the faulting page.

Extend the page donation logic to encode the guest handle and gfn
alongside the owner information in the host stage-2 pte.

Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Tested-by: Mostafa Saleh <smostafa@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Link: https://patch.msgid.link/20260330144841.26181-25-will@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>