libvirt: open RBD in read-only mode for read-only operations
libvirt: open RBD in read-only mode for read-only operations

The RBDDriver.size() and RBDDriver.parent_info() methods are read-only
operations, so open the RBDVolumeProxy connection for them in
read-only mode.

TrivialFix
Change-Id: I701ea8aca1938f38c3f6cc55d8ff6b56160f4821

rahulunair/nova,vmturbo/nova,mahak/nova,Juniper/nova,gooddata/openstack-nova,klmitch/nova,mahak/nova,hanlind/nova,mikalstill/nova,rajalokan/nova,vmturbo/nova,jianghuaw/nova,mikalstill/nova,jianghuaw/nova,Juniper/nova,gooddata/openstack-nova,alaski/nova,cloudbase/nova,openstack/nova,openstack/nova,alaski/nova,vmturbo/nova,sebrandon1/nova,sebrandon1/nova,rajalokan/nova,cloudbase/nova,rahulunair/nova,gooddata/openstack-nova,gooddata/openstack-nova,jianghuaw/nova,rahulunair/nova,openstack/nova,phenoxim/nova,hanlind/nova,Juniper/nova,phenoxim/nova,cloudbase/nova,mahak/nova,rajalokan/nova,mikalstill/nova,klmitch/nova,jianghuaw/nova,sebrandon1/nova,Juniper/nova,rajalokan/nova,klmitch/nova,klmitch/nova,hanlind/nova,vmturbo/nova