KVM: arm64: Hide CNTHV_*_EL2 from userspace for nVHE guests

KVM: arm64: Hide CNTHV_*_EL2 from userspace for nVHE guests

Although we correctly UNDEF any CNTHV_*_EL2 access from the guest
when E2H==0, we still expose these registers to userspace, which
is a bad idea.

Drop the ad-hoc UNDEF injection and switch to a .visibility()
callback which will also hide the register from userspace.

Fixes: 0e45981028550 ("KVM: arm64: timer: Don't adjust the EL2 virtual timer offset")
Signed-off-by: Marc Zyngier <maz@kernel.org>