A few of the qemu test cases assume that a dmi-to-pci-bridge will
always be added at index 1, and so they omit it from the input data
even though a pci-bridge is present at index 2, e.g.:
<controller type='pci' index='0' model='pcie-root'/>
<controller type='pci' index='2' model='pci-bridge'/>
Support for this odd practice was discussed on libvir-list and we
decided that the complex code required to make this continue was not
worth the headache of maintaining. So instead, this patch modifies the
test cases to manually add a dmi-to-pci-bridge at index 1 (since an
upcoming patch is going to eliminate the unconditional adding of
dmi-to-pci-bridge).
Because the auto-add was placing the dmi-to-pci-bridge later in the
list (even though it has a lower index) the test output is also
updated to take account for the new order (which puts the pci
controllers in index-order)
When the user doesn't specify any model for a USB controller,
we use an architecture-dependent default, but we don't reflect
it in the guest XML.
Pick the default USB controller model when parsing the guest
XML instead of when creating the QEMU command line, so that
our choice is saved back to disk.
