KVM: x86 emulator: Use emulator_write_emulated and not emulator_write_std

emulator_write_std() is not implemented, and calling write_emulated should
work just as well in place of write_std.

Fixes emulator failures with the push r/m instruction.

Signed-off-by: Amit Shah <amit.shah@qumranet.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>