Simplify PCI hostdev prepare/re-attach using a pciDeviceList type

Simplify PCI hostdev prepare/re-attach using a pciDeviceList type

The qemuPrepareHostDevices() and qemuDomainReAttachHostDevices()
functions are clutter with a bunch of calls to pciGetDevice() and
pciFreeDevice() obscuring the basic logic.

Add a pciDeviceList type and add a qemuGetPciHostDeviceList() function
to build a list from a domain definition. Use this in prepare/re-attach
fto simplify things and eliminate the multiple pciGetDevice calls.

This is especially useful because in the next patch we need to iterate
the hostdevs list a third time and we also need a list type for keeping
track of active devices.

* src/pci.[ch]: add pciDeviceList type and also a per-device 'managed'
  property

* src/libvirt_private.syms: export the new functions

* src/qemu_driver.c: add qemuGetPciHostDeviceList() and re-write
  qemuPrepareHostDevices() and qemuDomainReAttachHostDevices() to use it