Xen support

Xen support

Support launching helper domain on top of Xen hypervisor under PV mode.

Helper VM becomes hypervisor aware by passing hypervisor=$HYPERVISOR
in the kernel command line and thus acts accordingly (which logging
devices to use, what disk/floppy devices to expect, etc)

We separate all hypervisor specific functions and vars and source them
according to $HYPERVISOR env var.

Due to the fact that multiconsole for PV domains is supported only after
linux kernel 3.2 (Stefano Stabellini tree - branch 3.2-multiconsole-2
git://xenbits.xen.org/people/sstabellini/linux-pvhvm.git) all
output/error report and logging is done via hvc0.

Helper domain is debootstraped with 2 kernels (linux-image-amd64,
linux-image-xen-amd64) and is booted accordingly.

Floppy is not supported in PV so floppy is passed as another disk.

The images created can run on top of xen-hvm and xen-pvm hypervisor.

Change logging to start with HELPER_MONITOR_ in order to parse it when
having only one console.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>