vfio-pci: Enable PCIe extended config space

We don't know pre-init time whether the device we're exposing is PCIe
or legacy PCI.  We could ask for it to be specified via a device
option, but that seems like too much to ask of the user.  Instead we
can assume everything will be PCIe, which makes PCI-core allocate
enough config space.  Removing the flag during init leaves the space
allocated, but allows legacy PCI devices to report the real device
config space size to rest of Qemu.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
