libvirt: persist lxc attached volumes across reboots and power down

Currently, attached volumes to a LXC instance will be lost, when
hard rebooting or powering it down.
Adjusting the get_storage_config() to include LXC volumes in the
domain configuration.
The change will add all volumes to the instance config, a part from
the root device mapping.
When (re)creating a LXC instance, get_bdms_to_connect() generator
from block_device module, will be used to exclude the root device
mapping for a domain configuration.

Co-authored-by: Sam Morrison<sorrison@gmail.com>
Closes-Bug: #1269990
Change-Id: I0aa227ede5fca065c6da15506d8cc4391d993e20
Merge "libvirt: persist lxc attached volumes across reboots and power down"