libxl: add save/restore support for qemu-xen in stubdomain

libxl: add save/restore support for qemu-xen in stubdomain

Rely on a wrapper script in stubdomain to attach relevant consoles to
qemu.  The save console (1) must be attached to fdset/1.  When
performing a restore, $STUBDOM_RESTORE_INCOMING_ARG must be replaced on
the qemu command line by "fd:$FD", where $FD is an open file descriptor
number to the restore console (2).

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Address TODO in dm_state_save_to_fdset: Only remove savefile for
non-stubdom.
Use $STUBDOM_RESTORE_INCOMING_ARG instead of fd:3 and update commit
message.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>