python/machine: Change default timeout to 30 seconds

3 seconds is too short for some tests running inside busy VMs. Build it out to
a rather generous 30 seconds to find out conclusively if there are more severe
problems in the merge/CI tests.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-id: 20200720160252.104139-2-jsnow@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>