KVM: x86 emulator: Add std and cld instructions (opcodes 0xfc-0xfd)

This adds the std and cld instructions to the emulator.

Encountered while running the BIOS with invalid guest
state emulation enabled.

Signed-off-by: Mohammed Gamal <m.gamal005@gmail.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>