This goes further in the direction of d2d2ad8 "pci: get device class
out of libvirt xml".

PCI device function is just one hex digit.

The issue was about device with segment part exceeding 16 bits, which
technically is not spec compliant. Adjust the test to be about a device
that was problematic initially.

QubesOS/qubes-issues#6932

Qrexec service file is supposed to be a proper executable. If it's a
script, it need a shebang. Lacking that, it sometimes will work (qrexec
does call services via shell in some situations), but is unreliable.

PCI SBDF may change if _another_ device is added/removed from the
system. Use PCI device path to avoid this issue. The PCI device path is
more or less sysfs path, but with bus number behind bridges replaced
with relative bus number to bridge's secondary bus number. Each
component of the path is separated with '-', as that is allowed in
qrexec arguments.

For example device 0000:c1:00.0 behind bridge at 0000:00:08.1 will get
path 0000_00_08.1-00_00.0 (assuming bridge 0000:00:08.1 has secondary
bus at 0xc1). Resolving device path requires access to the system's
sysfs.

As a side effect, this adds also support for multi-segment systems, PCI
segment is no longer hardcoded to 0000.

PCIDevice object will automatically resolve old device identifiers,
which partially covers migration path. The other part is converting BDF
to device path when loading device assignments.

Tests include subset of sysfs from a real system that has multiple
bridges.

TODO:
- consider still allowing attaching via SBDF, or otherwise expose function
  to translate SBDF to PCI path (an Admin API call?)
- fix salt listing/attaching devices to sys-net/sys-usb - depending on
  the above

Fixes QubesOS/qubes-issues#8681

QubesOS/qubes-issues#8681

* pci-path:
  doc: document new PCI devices addressing
  ext/pci: implement PCI path for device identification
  Make black happy about qubes/utils.py
  tests: adjust test for unsupported device
  tests: fix invalid device BDF
  ext/pci: get PCI class from libvirt xml

* origin/pr/675:
  tests: fix creating test qrexec services

* origin/pr/674:
  tests: only close watch fd for LocalVM

Pull request description:

```
======================================================================
ERROR: qubes.tests.integ.misc/TC_10_RemoteVM_fedora-41-xfce/test_000_full_connect
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.13/site-packages/qubes/tests/__init__.py", line 891, in cleanup_app
    self.remove_test_vms()
    ~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib/python3.13/site-packages/qubes/tests/__init__.py", line 1216, in remove_test_vms
    self.remove_vms(
    ~~~~~~~~~~~~~~~^
        [
        ^
    ...<9 lines>...
        ]
        ^
    )
    ^
  File "/usr/lib/python3.13/site-packages/qubes/tests/__init__.py", line 1187, in remove_vms
    self._remove_vm_qubes(vm)
    ~~~~~~~~~~~~~~~~~~~~~^^^^
  File "/usr/lib/python3.13/site-packages/qubes/tests/__init__.py", line 1029, in _remove_vm_qubes
    del app.domains[vm.qid]
        ~~~~~~~~~~~^^^^^^^^
  File "/usr/lib/python3.13/site-packages/qubes/app.py", line 568, in __delitem__
    self.app.fire_event("domain-delete", vm=vm)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/qubes/events.py", line 200, in fire_event
    sync_effects, async_effects = self._fire_event(
                                  ~~~~~~~~~~~~~~~~^
        event, kwargs, pre_event=pre_event
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/usr/lib/python3.13/site-packages/qubes/events.py", line 169, in _fire_event
    effect = func(self, event, **kwargs)
  File "/usr/lib/python3.13/site-packages/qubes/tests/__init__.py", line 883, in close_qdb_on_remove
    if vm._qdb_connection_watch is not None:
       ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'RemoteVM' object has no attribute '_qdb_connection_watch'
```

Related to #669.

QubesOS/qubes-issues#9015

* origin/pr/668:
  doc: add qubes.vm.remotevm
  Introduce RelayVM property for RemoteVM
  Introduce RemoteVM class

Pull request description:

QubesOS/qubes-issues/issues/9015