cpu: add CPU_RESOLVING_TYPE macro

cpu: add CPU_RESOLVING_TYPE macro

it will be used for providing to cpu name resolving class for
parsing cpu model for system and user emulation code.

Along with change add target to null-machine tests, so
that when switch to CPU_RESOLVING_TYPE happens,
it would ensure that null-machine usecase still works.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu> (m68k)
Acked-by: David Gibson <david@gibson.dropbear.id.au> (ppc)
Acked-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de> (tricore)
Message-Id: <1518000027-274608-4-git-send-email-imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
[ehabkost: Added macro to riscv too]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>