libvirt: allow passing 'os_type' property to glance

Currently, libvirt driver doesn't pass the 'os_type' property to
glance. Thus all snapshots created don't have this property. Other
feature depend on 'os_type'(such as mkfs for ephemeral devices)
doesn't work.

This patch fixes this by passing the 'os_type' attribute to glance
as image properties. None value in metadata properties would not be
passed to glance, thus this patch has no side effect on situation
when instance has no 'os_type' attribute set.

Bug 1214372

Change-Id: I2bab617f6c1e8866650ca7dc5178b640738c9ae6
Merge "libvirt: allow passing 'os_type' property to glance"