As we start/shutdown guests, or hotplug/hot-unplug devices, we can add
or delete devices as appropriate from a list of active devices.

Then, in pciReset(), we can use this to determine whether its safe to
reset a device as a side effect of resetting another device.

* src/qemu_conf.h: add activePciHostdevs to qemud_driver

* src/qemu_driver.c: maintain the activePciHostdevs list, and pass it
  to pciResetDevice()

* src/pci.[ch]: pass the activeDevs list to pciResetDevice() and use
  it to determine whether a Secondary Bus Reset is safe