KVM: x86 emulator: Add wrappers for easily defining opcodes

Once 'struct opcode' grows, its initializer will become more complicated.
Wrap the simple initializers in a D() macro, and replace the empty initializers
with an even simpler N macro.

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