virsh: Elide backslash-newline in batch mode

virsh: Elide backslash-newline in batch mode

The previous patch made it possible to split multiple commands by
adding newline, but not to split a long single command. The sequence
backslash-newline was being used as if it were a quoted newline
character, rather than completely elided the way the shell does.

Again, add more tests, although this time it seems more like I am
suffering from a leaning-toothpick syndrome with all the \.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>