s390x/pci: acceleration for getting S390pciState

There are a number of places where the code needs to get the instance
of S390pciState. It calls object_resolve_path() every time. This
wastes a lot of time and leads to low performance. Thus we add
s390_get_phb() to improve it.

Because we always have a phb, we remove all return checkings in the
callers and add an assert in s390_get_phb() to make sure that phb is
getted successfully.

Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
