run: Introduce new options, change virt-test default behavior

OK guys, out of practicality, I've upgraded this pull request to showcase the change on virt-test behavior we've been discussing so far on the ML (thread https://www.redhat.com/archives/virt-test-devel/2013-December/msg00002.html).

This changes the virt-test defaults to safer options, that will give us the notion that VMs are always rebooted and their disks are restored from a pristine JeOS image. People wanting to save time are welcome to use the time saving options also introduced by these patches.

@adereis, you might be interested in taking a look/testing this.