xl: Allow shutdown wait for domain death

xl: Allow shutdown wait for domain death

`xl shutdown -w` waits for the first of either domain shutdown or death.
Shutdown is the halting of the guest operating system, and death is the
freeing of domain resources.

Allow specifying -w multiple times to wait for only domain death.  This
is useful in scripts so that all resources are free before the script
continues.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Paul Durrant <paul@xen.org>