libxl_pci: Constify arg `pcidev' of libxl__device_pci_add_xenstore

libxl_pci: Constify arg `pcidev' of libxl__device_pci_add_xenstore

libxl__device_pci_add_xenstore doesn't modify `pcidev', so it can be
constified. Also, we don't need pcidev_saved anymore, so remove the
saved copy. (device_add_domain_config is going to make it's own copy
anyway.)

To achieve this, constify pcidev in all functions that
libxl__device_pci_add_xenstore calls.

No functional changes.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>