target/arm: Update get_a64_user_mem_index for VHE

target/arm: Update get_a64_user_mem_index for VHE

The EL2&0 translation regime is affected by Load Register (unpriv).

The code structure used here will facilitate later changes in this
area for implementing UAO and NV.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200206105448.4726-36-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>