s390x/pci: enforce zPCI state checking

Current code uses some fields combinatorially to indicate the state of
a s390 pci device. This patch introduces device states in order to make
the code more readable and more logical.

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>
