KVM: arm64: Instantiate guest stage-2 page-tables at EL2

Extend the initialisation of guest data structures within the pKVM
hypervisor at EL2 so that we instantiate a memory pool and a full
'struct kvm_s2_mmu' structure for each VM, with a stage-2 page-table
entirely independent from the one managed by the host at EL1.

The 'struct kvm_pgtable_mm_ops' used by the page-table code is populated
with a set of callbacks that can manage guest pages in the hypervisor
without any direct intervention from the host, allocating page-table
pages from the provided pool and returning these to the host on VM
teardown. To keep things simple, the stage-2 MMU for the guest is
configured identically to the host stage-2 in the VTCR register and so
the IPA size of the guest must match the PA size of the host.

For now, the new page-table is unused as there is no way for the host
to map anything into it. Yet.

Tested-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221110190259.26861-20-will@kernel.org