KVM: arm64: Add infrastructure to create and track pKVM instances at EL2

Introduce a global table (and lock) to track pKVM instances at EL2, and
provide hypercalls that can be used by the untrusted host to create and
destroy pKVM VMs and their vCPUs. pKVM VM/vCPU state is directly
accessible only by the trusted hypervisor (EL2).

Each pKVM VM is directly associated with an untrusted host KVM instance,
and is referenced by the host using an opaque handle. Future patches
will provide hypercalls to allow the host to initialize/set/get pKVM
VM/vCPU state using the opaque handle.

Tested-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Co-developed-by: Will Deacon <will@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
[maz: silence warning on unmap_donated_memory_noclear()]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221110190259.26861-13-will@kernel.org