Add instance_type_get() to virt api

...and remove the use of instance['extra_specs'] from the libvirt and
baremetal virt drivers. Also remove the hack in instance_update()
which places them there in the first place.

Fixes bug 1133572

Change-Id: I39e9fabb28b48dc52ec47f58d76b0bf2c6ee0204
Merge "Add instance_type_get() to virt api"