All qemu versions we support have QEMU_CAPS_DEVICE, so checking
for it is redundant. Remove the usage.

The code diff isn't clear, but all that code is just inindented
with no other change.

Test cases that hit qemuDomainAssignAddresses but don't have
infrastructure for specifying qemuCaps values see lots of
churn, since now PCI addresses are in the XML output.