KVM: arm64: selftests: Fix misleading comment about virtual timer encoding

KVM: arm64: selftests: Fix misleading comment about virtual timer encoding

The userspace-visible encoding for CNTV_CVAL_EL0 and CNTVCNT_EL0
have been swapped for as long as usersapce has had access to the
registers. This is documented in arch/arm64/include/uapi/asm/kvm.h.

Despite that, the get_reg_list test has unhelpful comments indicating
the wrong register for the encoding.

Replace this with definitions exposed in the include file, and
a comment explaining again the brokenness.

Signed-off-by: Marc Zyngier <maz@kernel.org>