vmspawn: multifunction-pack pcie-root-ports on pcie.0 (#42077)

The pre-allocated pcie-root-port block in run_virtual_machine() places
every port directly on pcie.0 with an auto-assigned PCI address. A
minimal VM already costs 4 builtin + 10 hotplug spares = 14 pcie.0
slots, on top of 3 implicit virtio devices (virtio-rng-pci,
virtio-balloon, virtio-serial-pci) for another 3.

pcie.0 has 32 device-numbers; q35 reserves 0x00 (host bridge) and 0x1f
(ICH9 LPC), leaving ~30 auto-assignable slots. TEST-64-UDEV-STORAGE-
nvme_basic pushes 20 '-device nvme' lines through
$SYSTEMD_VMSPAWN_QEMU_EXTRA, which vmspawn does not see — total demand
14 + 3 + 20 = 37 > 30. Bus realization fails after QEMU's chardev has
already emitted the QMP greeting, and the monitor socket POLLHUPs while
we are mid-feature-probe, reported as 'QMP connection dropped during
feature probing'.

Pack the root ports as multifunction devices, 8 per pcie.0 device-
number (QEMU docs/pcie.txt:84, 117-120, 255-258). Function 0 of each
group carries multifunction=on; functions 1-7 ride the same slot via
addr=N.F. Each function remains independently hot-pluggable so vmspawn's
QMP device_add machinery is unaffected. 14 ports collapse to 2 pcie.0
slots; the nvme_basic budget becomes 2 + 3 + 20 = 25.

The chassis/slot properties (used for ACPI hotplug identity) stay as i+1
— they live in a uint8_t namespace independent of the PCI BDF and are
still unique. Base PCI slot 0x10 sits above the auto-assigned virtio
devices (which land at 0x01-0x03 in config order) and below the q35 LPC
reservation at 0x1f.

While here, rebuild the slot-count formula to match what
assign_pcie_ports() actually allocates. The +1 'SCSI controller' term
was bogus — virtio-scsi-pci comes from the hotplug-spares pool via
hotplug_port_owner[] in vmspawn-qmp.c, never from a builtin port (see
the comment in assign_pcie_ports()). The +1 'network' and +1 'vsock'
terms are now conditional on arg_network_stack and use_vsock. Bind
volumes were missing entirely. And the per-drive accounting now mirrors
assign_pcie_ports()'s skip-SCSI behaviour: non-SCSI drives (root +
extras + bind volumes) take one builtin port each, SCSI drives take none
— they share a controller drawn from the hotplug pool at device-add
time. Tighten the cap from UINT8_MAX to 192 (24 packed device-numbers ×
8) so we cannot claim more than 24 slots on pcie.0 regardless of how
many extras/runtime-mounts a caller asks for.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>