Add Haskell parser for "xm uptime"

In order to fetch precise information about the uptime of the VMs
running in Xen, we need to analyze the output of the "xm uptime" 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>