virsh: Don't clear old connection if 'connect $uri' fails

virsh: Don't clear old connection if 'connect $uri' fails

    virsh # list --all
     Id    Name                           State
    ----------------------------------------------------
     1     test                           running

    virsh # connect frob
    error: Failed to connect to the hypervisor
    error: no connection driver available for frob

    virsh # list --all
    error: failed to connect to the hypervisor
    error: no valid connection
    error: no connection driver available for frob

Seems sensible IMO to just not clear out the old connection state
until the new virConnectOpen succeeds.

https://bugzilla.redhat.com/show_bug.cgi?id=829160