libvirt: abort init_host method on libvirt that is too old

Nova has a declared minimum supported libvirt and the init_host
method will log an error message if it sees an older version is
installed. Unfortunately people are frequently not seeing this
error message and then filing bugs about things that are broken
on older libvirt versions.

Turn the log message into an exception so that nova-compute
process startup is completely aborted so people will see the
problem immediately rather than some unpredictable time later.

Closes-bug: #1376651
Change-Id: I87d5a41145b05cc2be3559ab528497fe8355ba85
Merge "libvirt: abort init_host method on libvirt that is too old"