Revert "s390x/ccw: create s390 phb conditionally"

This reverts commit d32bd032d8fde41281aae34c16a4aa97e9acfeac.

Turns out that old QEMUs always created a pci host bridge
and for many CPU models the migration from old QEMUs to new
QEMUs will fail with
qemu-system-s390x: Unknown savevm section or instance 'PCIBUS' 0
qemu-system-s390x: load of migration failed: Invalid argument

As a quick fix we will revert the commit and always create the
pci host bridge.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
[fixed revert to keep the comment fixup, added a comment in the code]
Cc: Cornelia Huck <cohuck@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Message-Id: <20170928131831.81393-1-borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
