virt-test: Extend the bootstrap procedure to accommodate various OS images.

The bootstrap procedure was hardcoded to use the JeOS image. If the JeOS
repository is unreachable the command will return an error. To use a specific OS
at bootstrap its name should be given in the run command using the --guest-os
parameter:
e.g. ./run -t qemu --bootstrap --guest-os=Enea.4.0.x86_64

Signed-off-by: Petre Pircalabu <petre.pircalabu@enea.com>