KVM: x86 emulator: Add cmp al, imm and cmp ax, imm instructions (ocodes 3c, 3d)

Add decode entries for these opcodes; execution is already implemented.

Signed-off-by: Guillaume Thouvenin <guillaume.thouvenin@ext.bull.net>
Signed-off-by: Avi Kivity <avi@qumranet.com>