drivers/virt: pkvm: Hook up mem_encrypt API using pKVM hypercalls

If we detect the presence of pKVM's SHARE and UNSHARE hypercalls, then
register a backend implementation of the mem_encrypt API so that things
like DMA buffers can be shared appropriately with the host.

Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240830130150.8568-5-will@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>