Fix the xenserver-install-wizard

The xenserver-install-wizard tries to start xapi and use the XenAPI to configure PIFs and SRs. The tricky bit is that we aren't running xen yet -- since we haven't rebooted -- and therefore xenstore and xenctrl aren't running.

This pull request causes squeezed to (i) retry if the xenstore connection is refused instead of exitting; and (ii) read domain zero's memory from /proc/meminfo rather than Xenctrl.