scsi-disk: add scsi-block for device passthrough

scsi-disk: add scsi-block for device passthrough

scsi-block is a new device that supports device passthrough of Linux
block devices (i.e. /dev/sda, not /dev/sg0).  It uses SG_IO for commands
other than I/O commands, and regular AIO read/writes for I/O commands.
Besides being simpler to configure (no mapping required to scsi-generic
device names), this removes the need for a large bounce buffer and,
in the future, will get scatter/gather support for free from scsi-disk.

Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>