libxl: remove get_all_assigned_devices() from libxl_pci.c

libxl: remove get_all_assigned_devices() from libxl_pci.c

Use of this function is a very inefficient way to check whether a device
has already been assigned.

This patch adds code that saves the domain id in xenstore at the point of
assignment, and removes it again when the device id de-assigned (or the
domain is destroyed). It is then straightforward to check whether a device
has been assigned by checking whether a device has a saved domain id.

NOTE: To facilitate the xenstore check it is necessary to move the
      pci_info_xs_read() earlier in libxl_pci.c. To keep related functions
      together, the rest of the pci_info_xs_XXX() functions are moved too.

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