on demand qemu screendump mechanism

[lkocman]

Autotest itself supports "screendumps" from qemu. These screendumps might be useful for determining why remote-viewer failed to connect and so on ...

rv_connect.py could use this for checking whenever password has been passed to dialog. Which means take screendump from vm2 right before passing sendkey("\n")

reference: client/virt/virt_env_process.py:def _take_screendumps(test, params, env):

If possible then make this function available to everybody (e.g. place it to virt_utils.py)

[lkocman]

screenshots are available (if not redefined) in results folder and transformed from .ppm to .jpg right after at the end of testing.

[vehrlich]

I'll take this issue.

[vehrlich]

After discussion with LMR screendump(self, file) from vm class does what we want and there is no need to change the framework (with moving method from virt_env_process)