KVM: arm64: Expose unshare hypercall to the host

Introduce an unshare hypercall which can be used to unmap memory from
the hypervisor stage-1 in nVHE protected mode. This will be useful to
update the EL2 ownership state of pages during guest teardown, and
avoids keeping dangling mappings to unreferenced portions of memory.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211215161232.1480836-14-qperret@google.com