Revert "arm64: KVM: Hide PMU from guests when disabled"

Commit 0c0543a1

 breaks migration and
introduces a regression with existing userspace because it introduces an
ordering requirement of setting up all VCPU features before writing ID
registers which we didn't have before.

Revert this commit for now until we have a proper fix.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>