move the responsibility of creating screenshots into the backend

The qemu backend will merge the 2 threads we have
add the screenshot tick right in the qemu loop
Merge pull request #70 from os-autoinst/coolo_vnc

integrate the screenshot loop into the qemu managment loop
very small optimizations

- don't reread the current screenshot
- don't create diff images, we never really used them

add timing to the search_template function
Merge pull request #71 from os-autoinst/coolo_vnc

very small optimizations
replace test-consoletest_finish-1 to generic-desktop
Merge pull request #72 from nilxam/replace_generic_desktop

replace test-consoletest_finish-1 to generic-desktop
fix username and password defaults

defaults didn't work anymore after switching to vars. Now use the same
values also for upgrade to be able to share previously installed openqa
images.

also allow to override username in password just in case.
add backend/VNC.pm to install list
Merge pull request #73 from lnussel/master

fix username and password defaults
we don't need no crypt any more