Update server_diagnostics to use instance object

Update the server_diagnostics API to get an instance object and then pass it
through the system down to the virt driver.

Converting the get_diagnostics() call in the compute manager also updates one of
the driver.get_info() calls to take an instance object.  Most had already been
converted.  There was one other stray case in the compute manager where it was
still taking an instance primitive, so this patch hits that one too to finish
the conversion of get_info() while we're at it.

Part of blueprint virt-objects

Change-Id: I20b324039c20f5b1fde4d55df2d679b7fb094573
Merge "Update server_diagnostics to use instance object"