libxl: remove unnecessary check from libxl__device_pci_add()

libxl: remove unnecessary check from libxl__device_pci_add()

The code currently checks explicitly whether the device is already assigned,
but this is actually unnecessary as assigned devices do not form part of
the list returned by libxl_device_pci_assignable_list() and hence the
libxl_pci_assignable() test would have already failed.

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