qapi/common.py: move build_params into gen.py

qapi/common.py: move build_params into gen.py

Including it in common.py creates a circular import dependency; schema
relies on common, but common.build_params requires a type annotation
from schema. To type this properly, it needs to be moved outside the
cycle.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20201009161558.107041-18-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>