ANDROID: KVM: arm64: Add __pkvm_guest_share_host()

In preparation for allowing a protected guest to share individual pages
back with the host for the purposes of things like virtio buffers,
introduce __pkvm_guest_share_host() to take care of the associated
page-table updates and permission checking.

Signed-off-by: Will Deacon <will@kernel.org>
Bug: 209580772
Change-Id: I6279b565b89a961628628aa4b1b592fdd57696e4
Signed-off-by: Will Deacon <willdeacon@google.com>