[xen-tools] catching ctrl-c and reverting failed install

[Henning Sprang]

In xen-tools, when I write a broken role script, whoch hangs, and I
hit ctrl-c at some point, everything just stops.

As I've been busy some while with xen-tools now, I know what to look for.
But newer users might appreciate it to get some message like

"canceling - you can find the rest of the failed install at
/tmp/kjdsnslknc. Unmount this directory and delete the volume XXX
manually before trying again."

One could also argue that users would expect by default that the whole
transaction is reverted, all lvms or files removed.

Henning