interface: arm_ffa: Add VM availability messages am: 92dc0f18aa

Original change: https://android-review.googlesource.com/c/trusty/lib/+/3481597

Change-Id: I076430fd6f2900e48dd93784954cfff8d05d9190
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>