Functions like virDomainOpenConsole() and virDomainOpenChannel() accept
NULL as a dev_name parameter.  Try using alias for the error message if
dev_name is not specified.

Before:
  error: internal error: character device <null> is not using a PTY

After:
  error: internal error: character device serial0 is not using a PTY
Signed-off-by: NLuyao Huang <lhuang@redhat.com>