ANDROID: KVM: arm64: Expose hyp s1 pool stats in debugfs

Expose two new files in KVM's debugfs folder, hyp_s1_pool_size and
hyp_s1_pool_free which respectively report the total size in bytes of
the hypervisor stage-1 pool, and the current amount of free memory in
the same pool.

This should be especially useful to allow fine-tuning of the
kvm-arm.hyp_lm_size_mb cmdline parameter.

Bug: 273700914
Change-Id: Ice0829008b5a8e5457d5ea28b131d5e30f914551
Signed-off-by: Quentin Perret <qperret@google.com>