KVM error - Error starting domain: Network not found: no network with matching name 'default'

when i'm trying to net-start it, i'm getting the following error

# virsh net-start default error: 

Failed to start network default

so i've checked net-list

# virsh net-list --all
Name                 State      Autostart 
-----------------------------------------

((Error starting domain: Network not found: no network with matching name 'default'))

and it is clear that network is not found. 

Fix:

If it is missing, then the example XML config can be reloaded & activated

# virsh net-define /usr/share/libvirt/networks/default.xml
Network default defined from /usr/share/libvirt/networks/default.xml

# virsh net-autostart default
Network default marked as autostarted

# virsh net-start default
Network default started

# virsh net-list --all
Name                 State      Autostart 
-----------------------------------------
default              active     yes

Fixed.