virt-test: Bootstrap using other 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>