scsi-disk: Allow overriding SCSI INQUIRY removable bit

Provide the "removable" qdev property bit to override the SCSI INQUIRY
removable (RMB) bit for non-CDROM devices.  This will be used by USB
Mass Storage Devices, which sometimes have this guest-visible bit set
and sometimes do not.  They therefore requires a means for user
configuration.

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