scsi: fix warning

hw/scsi-bus.c:758: warning: ‘xfer’ may be used uninitialized in this
function

Isn't true, but older gcc versions (for example 4.1 as shipped in rhel5)
are not clever enougth to figure, so sprinkle in a default: line to make
them happy.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
