libvirtError: POST operation failed: xend_post: error from xen daemon: (xend.err 'Device 0 (vif) could not be connected. Could not find bridge device xenbr1')

Issue

virtual machines won't start with the following error:

    Traceback (most recent call last): 
      File "/usr/share/virt-manager/virtManager/engine.py", line 498, in run_domain 
        vm.startup() 
      File "/usr/share/virt-manager/virtManager/domain.py", line 573, in startup 
        self.vm.create() 
      File "/usr/lib64/python2.4/site-packages/libvirt.py", line 287, in create 
        if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self) 
    libvirtError: POST operation failed: xend_post: error from xen daemon: (xend.err 'Device 0 (vif) could not be connected. Could not find bridge device xenbr1')