kvm-unit-tests: Add a func to run instruction in emulator

Add a function trap_emulator to run an instruction in emulator.
Set inregs first (%rax is invalid because it is used as return
address), put instruction codec in alt_insn and call func with
alt_insn_length. Get results in outregs.

Signed-off-by: Arthur Chunqi Li <yzt356@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>