xen: stop using loop device in run.py

[wkozaczuk]

This patch eliminates the use of losetup in run.py when running OSv on Xen. Instead of creating a loop device pointing to qcow2 or raw image, run.py adds an entry to the xl config file pointing directly to the OSv disk image like so:

disk=['/home/wkozaczuk/projects/osv/build/last/usr.img,qcow2,hda,rw']

or:

disk=['/home/wkozaczuk/projects/osv/build/last/usr.raw,raw,hda,rw']

In addition, we also enhanced run.py to support 2nd disk when running OSv on Xen. Finally, when executing run.py with '--dry-run', one can see the content of the xl config file which can be useful for troubleshooting or manually running OSv using the xl utility.

Examples to run OSv on Xen:

./scripts/run.py -p xen #uses default qcow2 image build/last/usr.img

./scripts/run.py -nv -c 1 -p xen --script vif-bridge -b virbr0

./scripts/run.py -p xen --second-disk-image ./disk.raw

Fixes #285 Fixes #344