libvirt: improve the specification of network disks

Allow zero or more hosts, and make the name optional, since only a
host is needed to use nbd. This allows cinder to fully specify the
location of a sheepdog or rbd volume without depending on extra
configuration files, and lays the groundwork for supporting nbd.

Rename the internal attribute source_host to source_name to reflect
its usage in the libvirt xml, and add source_hosts and source_ports
attributes to store the new information.

Fixes: bug 1077817
blueprint better-libvirt-network-volume-support
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>

Change-Id: I8ac431751692e52ba0786768cea996388962922d
Merge "libvirt: improve the specification of network disks"