hw/scsi/scsi-disk.c: Add very basic support for injecting disk errors on read

On every read operation, the block driver will check the value of
/tmp/qemu-disk-inject to determine what to do. If the file contains "0",
the disk operations will continue normally. If the file contains a
positive integer, the read operation will fail. The positive integer
must be a C errno, defined in asm-generic/errno-base.h, that the QEMU
SCSI subsystem understands to avoid undetermined behavior.