KVM: arm64: Add hvc handler at EL2 for hypercalls from protected VMs

KVM: arm64: Add hvc handler at EL2 for hypercalls from protected VMs

Add a hypercall handler at EL2 for hypercalls originating from protected
VMs. For now, this implements only the FEATURES and MEMINFO calls, but
subsequent patches will implement the SHARE and UNSHARE functions
necessary for virtio.

Unhandled hypercalls (including PSCI) are passed back to the host.

Reviewed-by: Vincent Donnefort <vdonnefort@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/20260330144841.26181-29-will@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>