ANDROID: KVM: arm64: Support multiple FF-A partition buffers

Store the ffa rx/tx pair of buffers in the hypervisor VM structure
and introduce a helper to return the address of this buffers.
Move the definition of the kvm_ffa_buffers to the common pkvm
header.
No functional change is intended to the existing host ff-a
proxy.

Bug: 269285339
Bug: 278749606
Change-Id: Id56675a050783bb846c2f9b21e147e564aaf411f
Signed-off-by: Sebastian Ene <sebastianene@google.com>