libvirt: Extract method _guest_add_memory_balloon
libvirt: Extract method _guest_add_memory_balloon

Partial-Bug: #1494374
Change-Id: Ibf3bd59fb4ce5233c26c64ef70a0ab05094ce10d
Signed-off-by: Maciej Kucia <33c1fdf481c8e628d4c6db7ea8dc77f49f2fa5d7@kucia.net>

openstack/nova,gooddata/openstack-nova,klmitch/nova,mahak/nova,openstack/nova,vmturbo/nova,Juniper/nova,rahulunair/nova,jianghuaw/nova,Juniper/nova,Juniper/nova,gooddata/openstack-nova,jianghuaw/nova,klmitch/nova,mahak/nova,openstack/nova,jianghuaw/nova,vmturbo/nova,rahulunair/nova,vmturbo/nova,phenoxim/nova,gooddata/openstack-nova,vmturbo/nova,phenoxim/nova,klmitch/nova,mikalstill/nova,mikalstill/nova,klmitch/nova,rahulunair/nova,gooddata/openstack-nova,mikalstill/nova,mahak/nova,Juniper/nova,jianghuaw/nova