ANDROID: KVM: arm64: Allow to read missing donations for pKVM heap alloc

hyp_alloc_missing_donations() allows hyp_alloc users to know how much
pages should be donated in order to fullfill their last failed
allocation. Calling this function will also reset this counter.

Bug: 278749606
Bug: 273748186
Change-Id: I37ba3e6ef26373a87e0293f8137939be505edcac
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>