Add pretty_xml utils

Add pretty_xml utils

This util reads on stdin and outputs an indented/formatted xml.
It is equivalent to "xmllint --format -".

It will be used for MI trigger testing. For testing we will essentially
diff the output of the command against the expected output. While a
nicely formatted multi-line output is not necessary for a machine to
do the diff, the human that will have to debug it will surely appreciate
it.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ie1597644941c55ce3e59f7ff16f196ac36325179