KVM: x86 emulator: Rename 'cr2' to 'memop'

Previous patches have removed the dependency on cr2; we can now stop passing
it to the emulator and rename uses to 'memop'.

Signed-off-by: Sheng Yang <sheng.yang@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>