kvm: selftest: unify the guest port macros

Most of the tests are using the same way to do guest to host sync but
the code is mostly duplicated.  Generalize the guest port macros into
the common header file and use it in different tests.

Meanwhile provide "struct guest_args" and a helper "guest_args_read()"
to hide the register details when playing with these port operations on
RDI and RSI.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>