docs: xl.pod.1: introduction to FLASK

docs: xl.pod.1: introduction to FLASK

Add a simple introduction to FLASK to the xl man page, at the beginning
of the FLASK chapter. Link to the xsm-flask.txt document.
Currently FLASK, TMEM and PCI PASS-THROUGH are defined as =head2 so they
look like sub-chapters of VIRTUAL DEVICE COMMANDS. Make them =head1.

Based on a text written by Daniel De Graaf.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>