Since the upgrade from Leap 15.0 to Leap 15.1, I get the following error when spawning a VM from virt-manager:

Error starting domain: Requested operation is not valid: network 'default' is not active
Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 75, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 111, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/libvirtobject.py", line 66, in newfn
    ret = fn(self, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/domain.py", line 1422, in startup
    self._backend.create()
  File "/usr/lib64/python3.6/site-packages/libvirt.py", line 1089, in create
    if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
libvirt.libvirtError: Requested operation is not valid: network 'default' is not active

How to reproduce (not sure if it's really reproducible, I don't have a spare bare metal host to try it):
* install leap 15.0
* set up some vms using virt-manager
* update to leap 15.1
* shutdown workstation and turn it on again
* open virt-manager, right click on a vm and then "run"
* the popup error appears

Workaround:
sudo virsh net-start default

As you can see the network should start on boot:
$ sudo virsh net-info default
Name:           default
UUID:           4d29642d-c3bf-4e69-98a3-2edcf4f66aed
Active:         no
Persistent:     yes
Autostart:      yes
Bridge:         virbr0