The VM page reads the VM from libvirt throughout. When its host could
not be reached, the connection error replaced the page with the error
page and showed libvirt's message, which names the host and how it is
reached, to any user who may view the VM.
The page now checks Compute.status first. If the host is not connected,
it goes to the instance list with a message: administrators (superuser
or view_instances) get the host and the error, other users only that
the VM's host cannot be reached right now. The list shows the VM as
unknown.
This covers the page itself; actions on the VM still report libvirt's
error if the host drops. The detail page tests now mock a connected
host.
Checked against the test hosts: a real VM's page opens as before, a VM
on an unreachable compute leads to the list with the user's or the
administrator's message. The new tests fail on the previous code.
