scsi: Allow scsi_bus_legacy_add_drive() to set removable bit

scsi-disk devices may wish to override the removable bit.  Add support
for a qdev property on SCSI devices.  This is will be used by usb-msd.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
