libxl: workaround gcc 10.2 maybe-uninitialized warning

libxl: workaround gcc 10.2 maybe-uninitialized warning

It seems xlu_pci_parse_bdf has a state machine that is too complex for
gcc to understand. The build fails with:

    libxlu_pci.c: In function 'xlu_pci_parse_bdf':
    libxlu_pci.c:32:18: error: 'func' may be used uninitialized in this function [-Werror=maybe-uninitialized]
       32 |     pcidev->func = func;
          |     ~~~~~~~~~~~~~^~~~~~
    libxlu_pci.c:51:29: note: 'func' was declared here
       51 |     unsigned dom, bus, dev, func, vslot = 0;
          |                             ^~~~
    libxlu_pci.c:31:17: error: 'dev' may be used uninitialized in this function [-Werror=maybe-uninitialized]
       31 |     pcidev->dev = dev;
          |     ~~~~~~~~~~~~^~~~~
    libxlu_pci.c:51:24: note: 'dev' was declared here
       51 |     unsigned dom, bus, dev, func, vslot = 0;
          |                        ^~~
    libxlu_pci.c:30:17: error: 'bus' may be used uninitialized in this function [-Werror=maybe-uninitialized]
       30 |     pcidev->bus = bus;
          |     ~~~~~~~~~~~~^~~~~
    libxlu_pci.c:51:19: note: 'bus' was declared here
       51 |     unsigned dom, bus, dev, func, vslot = 0;
          |                   ^~~
    libxlu_pci.c:29:20: error: 'dom' may be used uninitialized in this function [-Werror=maybe-uninitialized]
       29 |     pcidev->domain = domain;
          |     ~~~~~~~~~~~~~~~^~~~~~~~
    libxlu_pci.c:51:14: note: 'dom' was declared here
       51 |     unsigned dom, bus, dev, func, vslot = 0;
          |              ^~~
    cc1: all warnings being treated as errors

Workaround it by setting the initial value to invalid value (0xffffffff)
and then assert on each value being set. This way we mute the gcc
warning, while still detecting bugs in the parse code.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>