tcg/i386: Check the size of instruction being translated

tcg/i386: Check the size of instruction being translated

This fixes the bug: 'user-to-root privesc inside VM via bad translation
caching' reported by Jann Horn here:
https://bugs.chromium.org/p/project-zero/issues/detail?id=1122

Reviewed-by: Richard Henderson <[email protected]>
CC: Peter Maydell <[email protected]>
CC: Paolo Bonzini <[email protected]>
Reported-by: Jann Horn <[email protected]>
Signed-off-by: Pranith Kumar <[email protected]>
Message-Id: <20170323175851[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>