bug #2859: fix VNC in Xen PV deployment file

(cherry picked from commit 8c3e2057c7cda417ef29ce669e1e14bf31ddbe01)
bug #2868: fix OpenNebula.pool_page size problem

Now OpenNebula.pool_page_size is a module method and the value is stored
in the class variable @@pool_page_size

(cherry picked from commit 21b3ac286934864d7c27bcc9fe8c4815b07e1bdb)