openstack: Store images in /var/lib/xapi instead of /var/run

/var/run is a tmpfs on Ubuntu, so if images can be created at all
(the tmpfs is small) they would be lost on reboot.

/var/lib/xapi is created by the xapi package, so it's safe to assume
it will be present.

Signed-off-by: Euan Harris <euan.harris@citrix.com>