qemu: assign e1000e network devices to PCIe slots when appropriate

qemu: assign e1000e network devices to PCIe slots when appropriate

The e1000e is an emulated network device based on the Intel 82574,
present in qemu 2.7.0 and later. Among other differences from the
e1000, it presents itself as a PCIe device rather than legacy PCI. In
order to get it assigned to a PCIe controller, this patch updates the
flags setting for network devices when the model name is "e1000e".

(Note that for some reason libvirt has never validated the network
device model names other than to check that there are no dangerous
characters in them. That should probably change, but is the subject of
another patch.)

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1343094