Pass Instance object to get_spice_console virt api

Update the virt API to be defined as taking an Instance object.
Luckily, no real changes are needed in the virt drivers to make this
work.  While the API is changing anyway, pass the context in as well
since we have a TODO to add it to virt API methods when convenient.

Change-Id: I2b2f389aee3f2cee3bb1361958da797735c60e18
Merge "Pass Instance object to get_spice_console virt api"