target/arm: Make NV reads of CurrentEL return EL2

FEAT_NV requires that when HCR_EL2.NV is set reads of the CurrentEL
register from EL1 always report EL2 rather than the real EL.
Implement this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Miguel Luis <miguel.luis@oracle.com>