Adding blockdev_prefix to hypervisor options

Allows to install Red Hat based systems, for example Oracle Linux.
Tested with OEL.

The hypervisor by default offers a device named 'sda'.  If the SCSI
module is already loaded, the disk device can't be created due to naming
conflict, and the disk is not available.  A workaround is to modify the
initrd by removing the scsi driver from it.  This helps, but doesn't
allow to install the OS.

Red Hat's installer, anaconda, runs parted, which tries to execute a
check against /dev/sda and fails.  This makes anaconda think that the
disk is faulty, and not available.  The best way to work around this, is
to declare 'xvda' as the xen disk device.  Red Hat version of parted
package contains a patch which makes parted skip the SCSI test if device
name starts with 'xvd'.

This patch allows to pass -H xen-pvm:blockdev_prefix="xvd" and
successfully run the Red Hat installer.

Signed-off-by: Maciej Bliziński <blizinski@google.com>
[iustin@google.com: added the new parameter to XenHvm PARAMS]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>