ANDROID: KVM: arm64: Support pVM memory sharing with the hypervisor

Allow a protected guest VM to share memory pages with the hypervisor.
Return the hypervisor virtual address of the page being shared to the
caller to prevent additional walks of the guest s2 pagetable.

Bug: 269285339
Bug: 278749606
Change-Id: I18a43667f94debf660b88799bda276773144855a
Signed-off-by: Sebastian Ene <sebastianene@google.com>