ANDROID: KVM: arm64: Donate memory to protected guests

Instead of sharing memory with protected guests, which still leaves the
host with r/w access, donate the underlying pages so that they are
unmapped from the host stage-2.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Will Deacon <willdeacon@google.com>
Bug: 233587962
Change-Id: I3e0d1d31877acf3978e82350ebbe92136919507c