Qemu-Guest-Agent (QGA): use implicit PCI slot
Qemu-Guest-Agent (QGA): use implicit PCI slot

According to `doc/design-hotplug.rst`, explicit PCI slot management via
KVM runtime is for Disks and NICs only (see also comment on
`update_hvinfo()`). Current PCI slot assignment is calculated from KVM
runtime, not live query. This way, misuse of PCI slots, like QGA, are
not detected. While QMP `query-pci` would cover virtio-blk-pci and
virtio-net-pci, it does not cover the SCSI disk case. Keeping the fix
simple, let QGA use an implicit Qemu managed PCI slot (like spice agent
virtio-serial).

This fixes #1620

ganeti/ganeti,ganeti/ganeti,ganeti/ganeti