KVM: x86 emulator: simplify emulate_1op_rax_rdx()

emulate_1op_rax_rdx() is always called with the same parameters.  Simplify
by passing just the emulation context.

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