Delete unused 'virt_type' key from nodes list.

This key was added but not documented, and then superseded by the
pm_type parameter (for Ironic which supports heterogeneous drivers).

Remove it to avoid confusion.

Change-Id: I55fb277bc2d5917083e7c14c82a3eb351993ea13
Merge "Delete unused 'virt_type' key from nodes list."