xen-blkfront: Drop name and minor adjustments for emulated scsi devices

These were intended to avoid the namespace clash when representing
emulated IDE and SCSI devices. However that seems to confuse users
more than expected (a disk defined as sda becomes xvde).
So for now go back to the scheme which does no adjustments. This
will break when mixing IDE and SCSI names in the configuration of
guests but should be by now expected.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>