libvirt: Custom disk_bus setting is being lost on hard_reboot

Currently, a non-default disk_bus setting of and image will not be used,
when powering on an instance (or _hard_reboot).

hard_reboot() should use image_meta, if it exist, when constructing
the disk_info

Closes-Bug: #1255449

(cherry picked from commit 0fe3001d639df81ae4e5f77bcfc32bc1bffb014e)

Conflicts:
	nova/virt/libvirt/driver.py

Change-Id: I477f69c47515ad22165421b4d23ac865c7869873
Merge "libvirt: Custom disk_bus setting is being lost on hard_reboot" into stable/havana