Python: expose QEMUMachine's temporary directory

Python: expose QEMUMachine's temporary directory

Each instance of qemu.machine.QEMUMachine currently has a "test
directory", which may not have any relation to a "test", and it's
really a temporary directory.

Users instantiating the QEMUMachine class will be able to set the
location of the directory that will *contain* the QEMUMachine unique
temporary directory, so that parameter name has been changed from
test_dir to base_temp_dir.

A property has been added to allow users to access it without using
private attributes, and with that, the directory is created on first
use of the property.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210211220146.2525771-3-crosa@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>