arm64: KVM: Hide PMU from guests when disabled

Since commit 93390c0a

 ("arm64: KVM: Hide unsupported AArch64 CPU
features from guests") we can hide cpu features from guests. Apply
this to a long standing issue where guests see a PMU available, but
it's not, because it was not enabled by KVM's userspace.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>