arm64: RME: Allocate/free RECs to match vCPUs

The RMM maintains a data structure known as the Realm Execution Context
(or REC). It is similar to struct kvm_vcpu and tracks the state of the
virtual CPUs. KVM must delegate memory and request the structures are
created when vCPUs are created, and suitably tear down on destruction.

Signed-off-by: Steven Price <steven.price@arm.com>