KVM: arm64: Add protected_shared_mem statistic

When using nVHE in protected mode, protected memory can be between
host and a guest. Tracking this value is interesting from a debug
perspective, to identify potential leaks.

Keeping the count of memory sharing is easy, each share/unshare will return
to the host where the accounting will take place.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>