tools/hotplug: Add a "dummy" hotplug script for testing

tools/hotplug: Add a "dummy" hotplug script for testing

Testing the hotplug external script path at the moment involves
actually setting up one of the alternate datapaths (blktap, iscsi,
&c).  Simplify testing by making a script which does a simple loopback,
but still has a target that can't be used directly.

To use:

script=block-dummy,vdev=xvda,target=dummy:<file>

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Tested-by: George Dunlap <george.dunlap@citrix.com>