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"