Libvirt driver create images even without meta

This patch allows the libvirt driver to call _create_image and
create instance directory and other needed images even when there is no
image metadata specified.

As an added bonus, this patch refactors the code of _create_image
method a bit to make it more clear to the reader when some of the images
will and will not be created, especially with regard to booting from
volume.

Fixes bug: 1124441
Fixes bug: 1131913
Fixes bug: 1123274

blueprint: improve-boot-from-volume

Change-Id: I5a028dc0585876d35be4fb86df3a423d89e054ee