avocado_qemu: tweak ssh connect method

The current implementation will crash if the connection fails as the
`time` module is not imported. Fix the import problem. While here,
tweaks the connection to wait progressively when the connection fails.

Signed-off-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[PMD: Reworded description]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210920204932.94132-5-willianr@redhat.com>