ANDROID: KVM: arm64: Handle guest FF-A map call

Map the guest FF-A buffers in the hypervisor address space and store the
address of the buffers in the global array indexed by the vm_handle.
Refactor the logic for the host without altering the original behaviour
and introduce a method for handling the mapping for the guest. Pass the
exit_code pointer to the guest handler to be able to signal to the host if
the page that we are trying to share needs to be faulted in or the vcpu
memcache needs to be topped up. If the map handler returns a negative
error code we get beack to the host without updating the vcpu context.
Move the pkvm_handle_empty_memcache and the __pkvm_memshare_page_req to
the common header to be able to call them from the ffa logic.

Bug: 269285339
Bug: 278749606
Change-Id: I8ccb19c35c2e06574bb889c94631b31135f97177
Signed-off-by: Sebastian Ene <sebastianene@google.com>