KVM: x86 emulator: Use a register for ____emulate_2op() destination

Most x86 two operand instructions allow the destination to be a memory operand,
but IMUL (for example) requires that the destination be a register.  Change
____emulate_2op() to take a register for both source and destination so we
can invoke IMUL.

Signed-off-by: Avi Kivity <avi@redhat.com>