Seems that recent updates has broken libvirt/virt-manager. I only use this vm occasionally so I don't know when it started exactly.
Unable to connect to libvirt qemu:///system.
internal error: client socket is closed
Libvirt URI is: qemu:///system
Traceback (most recent call last):
File "/usr/share/virt-manager/virtManager/connection.py", line 932, in _do_open
self._backend.open(cb, data)
~~~~~~~~~~~~~~~~~~^^^^^^^^^^
File "/usr/share/virt-manager/virtinst/connection.py", line 184, in open
self._get_caps() # cache and log capabilities
~~~~~~~~~~~~~~^^
File "/usr/share/virt-manager/virtinst/connection.py", line 107, in _get_caps
capsxml = self._libvirtconn.getCapabilities()
File "/usr/lib/python3/dist-packages/libvirt.py", line 4699, in getCapabilities
raise libvirtError('virConnectGetCapabilities() failed')
libvirt.libvirtError: internal error: client socket is closed