KVM: x86 emulator: fix src, dst value initialization

Some operand fetches are less than the machine word size and can result in
stale bits if used together with operands of different sizes.

Signed-off-by: Nitin A Kamble <nitin.a.kamble@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>