Add pcihole64 element to root PCI controllers

Add pcihole64 element to root PCI controllers

<controller type='pci' index='0' model='pci-root'>
  <pcihole64 unit='KiB'>1048576</pcihole64>
</controller>

It can be used to adjust (or disable) the size of the 64-bit
PCI hole. The size attribute is in kilobytes (different unit
can be specified on input), but it gets rounded up to
the nearest GB by QEMU.

Disabling it will be needed for guests that crash with the
64-bit PCI hole (like Windows XP), see:
https://bugzilla.redhat.com/show_bug.cgi?id=990418