core: fix vm devices upgrade in 3.1

when upgrading from 3.0 to 3.1 vm devices are created,
and boot order is set to new devices,
currenly upgrade code doesn't take into consideration the old boot
order, in case there are more than one disks.

this patch use 'boot' column from images table to order the boot disk to
be first device handled, and then it will have the right boot order.

Change-Id: Ic83725ad44363c75cb6b4230c983383dcbd24d0f
Bug-Url: https://bugzilla.redhat.com/967268
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>