Run libvirt version check before making libvirt calls

[Matt Thompson]

I tried running Nova (Grizzly) w/ libvirt version 0.8 and noticed that no errors were logged about using an unsupported libvirt version. It looks like init_host in nova/virt/libvirt/driver.py makes libvirt calls before checking version (libvirt.virEventRegisterDefaultImpl() as an example) and if these calls aren't supported by the libvirt version installed then an exception will be raised prior to the error being logged.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/32734

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/32734
Committed: http://github.com/openstack/nova/commit/db0ac5ff995143992e306fe5246a2375473c8030
Submitter: Jenkins
Branch: master

commit db0ac5ff995143992e306fe5246a2375473c8030
Author: Matt Thompson <email address hidden>
Date: Wed Jun 12 15:20:02 2013 +0100

    Check libvirt version earlier

    Running libvirt 0.8 causes an exception to be thrown before
    has_min_version() is run, resulting in no error message about version
    being logged. This change moves the version check before the libvirt
    calls are made.

    Change-Id: Ifaae18c6686d72fb79fc32081c4087084ae8bbb9
    Fixes: bug #1190232