db: add logical_name to vm_device

This patch adds the logical_name column to the vm_device table,
the intention is to store in it the device logical name as retrieved
from the guest.

Change-Id: If9b5a8e771a05c1fe44834adfe998cf921f6ae52
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1063597
Signed-off-by: Liron Aravot <laravot@redhat.com>