xenapi: Emphasize EXT type SR installation
xenapi: Emphasize EXT type SR installation

This patch highlights the importance of installing an EXT type SR during
the XenServer installation. Also fix a typo.

Change-Id: I54f67f0d8748757db8e9cd6fc79418b76e8f2868

AlekhyaMallina-Vedams/openstack-manuals,saeki-masaki/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,gongysh2004/openstack-manual,potsmaster/openstack-manuals,gongysh2004/openstack-manual,AlekhyaMallina-Vedams/openstack-manuals,potsmaster/openstack-manuals,Akasurde/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,saeki-masaki/openstack-manuals,Akasurde/openstack-manuals