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

Change-Id: I477f69c47515ad22165421b4d23ac865c7869873
Update vnc virt driver api to take Instance object

Update the get_vnc_console() virt api method to take an Instance object
instead of a dict.  No real changes were needed in the drivers to make
this work.  While we're changing the API call, add the context, as well.

Change-Id: I2026ea725d61df4b88e7618f9fd4498c62843296
Merge "Update vnc virt driver api to take Instance object"
Merge "libvirt: Custom disk_bus setting is being lost on hard_reboot"