KVM: arm64: Add helpers to pin memory shared with the hypervisor at EL2

Add helpers allowing the hypervisor to check whether a range of pages
are currently shared by the host, and 'pin' them if so by blocking host
unshare operations until the memory has been unpinned.

This will allow the hypervisor to take references on host-provided
data-structures (e.g. 'struct kvm') with the guarantee that these pages
will remain in a stable state until the hypervisor decides to release
them, for example during guest teardown.

Tested-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221110190259.26861-9-will@kernel.org