core: protocol fall back for older vdsms

When we try to install new host which only supports xmlrpc and we have
cluster with compatibility level set to 3.5+ the code will fail.
Solution for this issue is attempting to connect by using jsonrpc and if
it fails falling back to xmlrpc.

Bug-Url: https://bugzilla.redhat.com/1149655
Change-Id: Ie6f48bec60b520c089f326f8c5e79aec288ff3d6
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>