docs/man: modify xl-pci-configuration(5) to add 'name' field to PCI_SPEC_STRING

docs/man: modify xl-pci-configuration(5) to add 'name' field to PCI_SPEC_STRING

Since assignable devices can be named, a subsequent patch will support use
of a PCI_SPEC_STRING containing a 'name' parameter instead of a 'bdf'. In
this case the name will be used to look up the 'bdf' in the list of assignable
(or assigned) devices.

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