Description of problem:
For ARM VMs virt-manager uses the same default NIC ("Hypervisor default"), i.e. PCI device rtl8139, as for x86 VMs. But that driver is not included in the standard Fedora ARM kernels.
When you execute the ARM VM by hand with "qemu-system-arm -net nic ..." then you can see that the default NIC for ARM is
name "smc91c111", bus System
for which the driver is included in the Fedora ARM kernel.
My knowledge of libvirt XML is limited, but I couldn't write the VM XML in such a way that I could define NIC type "smc91c111" on the bus "System", i.e. that libvirt would execute qemu-system-arm with the correct parameters. So the root cause for this problem might be libvirt, not virt-manager.
Version-Release number of selected component (if applicable):
There are issues with non x86 archs in libvirt and virt-manager. The main blockers are in libvirt though: in an attempt to have stable device addressing it gets the defaults wrong for most non-x86 archs. Reassigning there.
I posted a series upstream which fixes this issue:
Just closing as UPSTREAM since I'll probably forget to update this bug when the patches actually land.