pci: removed the is_express field since a uniform interface was inserted

according to Eduardo Habkost's commit fd3b02c8 all PCIEs now implement
INTERFACE_PCIE_DEVICE so we don't need is_express field anymore.

Devices that implements only INTERFACE_PCIE_DEVICE (is_express == 1)
or
devices that implements only INTERFACE_CONVENTIONAL_PCI_DEVICE (is_express == 0)
where not affected by the change.

The only devices that were affected are those that are hybrid and also
had (is_express == 1) - therefor only:
  - hw/vfio/pci.c
  - hw/usb/hcd-xhci.c
  - hw/xen/xen_pt.c

For those 3 I made sure that QEMU_PCI_CAP_EXPRESS is on in instance_init()

Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Yoni Bettan <ybettan@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>