Add Haskell parser for "xm list --long"

In order to fetch precise information about the status of the VMs running in
Xen, we need to analyze the output of the "xm list --long" command.

This commit adds the parser to do that, and its tests.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>