qemu: add capabilities flags related to SG_IO

qemu: add capabilities flags related to SG_IO

This patch adds two capabilities flags to deal with various aspects
of supporting SG_IO commands on virtio-blk-pci devices:

  QEMU_CAPS_VIRTIO_BLK_SCSI
    set if -device virtio-blk-pci accepts the scsi="on|off" option
    When present, this is on by default, but can be set to off to disable
    SG_IO functions.

  QEMU_CAPS_VIRTIO_BLK_SG_IO
    set if SG_IO commands are supported in the virtio-blk-pci driver
    (present since qemu 0.11 according to a qemu developer, if I
     understood correctly)