Bootstrap Python venv for tests

Bootstrap Python venv for tests

A number of QEMU tests are written in Python, and may benefit
from an untainted Python venv.

By using make rules, tests that depend on specific Python libs
can set that rule as a requirement, along with rules that require
the presence or installation of specific libraries.

The tests/requirements.txt is supposed to contain the Python
requirements that should be added to the venv created by check-venv.

Signed-off-by: Cleber Rosa <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Acked-by: Stefan Hajnoczi <[email protected]>
Acked-by: Wainer dos Santos Moschetta <[email protected]>
Reviewed-by: Caio Carrara <[email protected]>
Message-Id: <20181018153134[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>