libxl / libxlu: support 'xl pci-attach/detach' by name

libxl / libxlu: support 'xl pci-attach/detach' by name

This patch adds a 'name' field into the idl for 'libxl_device_pci' and
libxlu_pci_parse_spec_string() is modified to parse the new 'name'
parameter of PCI_SPEC_STRING detailed in the updated documention in
xl-pci-configuration(5).

If the 'name' field is non-NULL then both libxl_device_pci_add() and
libxl_device_pci_remove() will use it to look up the device BDF in
the list of assignable devices.

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