build: use proper pod for nested bulleted VIRSH_DEBUG list

Newer pod (hello rawhide) complains if you attempt to mix bullets
and non-bullets in the same list:

virsh.pod around line 3177: Expected text after =item, not a bullet

As our intent was to nest an inner list, we make that explicit to
keep pod happy.

* tools/virsh.pod (ENVIRONMENT): Use correct pod syntax.

(cherry picked from commit 00d69b4a)

tools/virsh.pod

@@ -2705,6 +2705,8 @@ of C<virsh>
 
 Turn on verbose debugging of virsh commands. Valid levels are
 
+=over 4
+
 =item * VIRSH_DEBUG=0
 
 DEBUG - Messages at ALL levels get logged
@@ -2725,6 +2727,8 @@ WARNING - Logs messages at levels WARNING and ERROR
 
 ERROR - Messages at only ERROR level gets logged.
 
+=back
+
 =item VIRSH_LOG_FILE=C<LOGFILE>
 
 The file to log virsh debug messages.