Fix incorrect implication about list options

The description of the list command seemed to suggest that it could
take a set of domains as an argument, which is not correct in the
current HEAD.  If virsh list is intended to take a list of domains,
then this patch should be NAK'd and a bug opened against virsh list.

Reported by hachi on #virt

v2:

Change language to include transient domains

Osier pointed out that transient domains are not defined, so what I
had originally proposed wasn't quite correct.

tools/virsh.pod

@@ -268,7 +268,7 @@ Inject NMI to the guest.
 
 =item B<list> [I<--inactive> | I<--all>]
 
-Prints information about one or more domains.  If no domains are
+Prints information about existing domains.  If no options are
 specified it prints out information about running domains.
 
 An example format for the list is as follows: