Revert "hw/pci: disable pci-bridge's shpc by default"

This reverts commit dc0ae767.

Disabling the shpc controller has an undesired side effect.
The PCI bridge remains with no attached devices at boot time,
and the guest operating systems do not allocate any resources
for it, leaving the bridge unusable. Note that the behaviour
is dictated by the pci bridge specification.

Revert the commit and leave the shpc controller even if is not
actually used by any architecture. Slot 0 remains unusable at boot time.

Keep shpc off for QEMU 2.9 machines.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>