Kvm qemu backend

[rmescandon]

Includes support for qemu backend in the snap. Some considerations:

• searches for a image in $SNAP_USER_DATA/.spread/qemu
• you can overwrite where to search the image by setting $SPREAD_QEMU_PATH env var
• the project to be tested must be in a path accessible by the snap (maybe this could be improved in future by using content interfaces or other mechanism)

You can, for instance, copy your project to /var/snap/spread/current/<your_project> and launch spread command from within there.