libxl: generalise 'driver_path' xenstore access functions in libxl_pci.c

libxl: generalise 'driver_path' xenstore access functions in libxl_pci.c

For the purposes of re-binding a device to its previous driver
libxl__device_pci_assignable_add() writes the driver path into xenstore.
This path is then read back in libxl__device_pci_assignable_remove().

The functions that support this writing to and reading from xenstore are
currently dedicated for this purpose and hence the node name 'driver_path'
is hard-coded. This patch generalizes these utility functions and passes
'driver_path' as an argument. Subsequent patches will invoke them to
access other nodes.

NOTE: Because functions will have a broader use (other than storing a
      driver path in lieu of pciback) the base xenstore path is also
      changed from '/libxl/pciback' to '/libxl/pci'.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Wei Liu <wl@xen.org>