accel/tcg: Avoid caching overwritten tlb entries

When installing a TLB entry, remove any cached version of the
same page in the VTLB.  If the existing TLB entry matches, do
not copy into the VTLB, but overwrite it.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>