ANDROID: KVM: arm64: Unshare pages from __unmap_stage2_range()

unmap_stage2_range(), as its name suggests, is called by KVM when
stage-2 mappings need to be removed (removing memslots, ...). Since the
host has now gained the ability to unshare pages with non-protected
guest, we can plumb this function using the recently introduced HVC,
hence paving the way for enabling more KVM features with np guests.

Bug: 227300338
Bug: 278749606
Change-Id: Ib750c912b6477001ba46a00c55fda79358b8f11e
Signed-off-by: Quentin Perret <qperret@google.com>