Libvirt: Don't let get_console_output crash on missing console file.

During a migration, the console file may be configured, even though
the file itself is not yet present. This patch prevents a
ProcessExecutionError which gets thrown by the chown if the file
doesn't exist.

Change-Id: Ifd75ae911fc264f8284ec3f2adbf1d73bc8c4aca
Related-Bug: #1292339
Merge "Libvirt: Don't let get_console_output crash on missing console file."