vfio: add pcie extended capability support

For vfio pcie device, we could expose the extended capability on
PCIE bus. due to add a new pcie capability at the tail of the chain,
in order to avoid config space overwritten, we introduce a copy config
for parsing extended caps. and rebuild the pcie extended config space.

Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
