xl: support for leaving domain paused after save

New feature to allow xl save to leave a domain paused after its
memory has been saved. This is to allow disk snapshots of domU
to be taken that exactly correspond to the memory state at save time.
Once the snapshot(s) have been taken or whatever, the domain can be
unpaused in the usual manner.

Usage:
    xl save -p <domid> <filespec>

Signed-off-by: Ian Murray <murrayie@yahoo.co.uk>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

docs/man/xl.pod.1

@@ -504,7 +504,7 @@ Pass VNC password to vncviewer via stdin.
 
 Saves a running domain to a state file so that it can be restored
 later.  Once saved, the domain will no longer be running on the
-system, unless the -c option is used.
+system, unless the -c or -p options are used.
 B<xl restore> restores from this checkpoint file.
 Passing a config file argument allows the user to manually select the VM config
 file used to create the domain.
@@ -515,6 +515,10 @@ file used to create the domain.
 
 Leave domain running after creating the snapshot.
 
+=item B<-p>
+
+Leave domain paused after creating the snapshot.
+
 =back
 
 =item B<sharing> [I<domain-id>]