Split XENVBD_REQUEST into unioned parts by BLKIF_OP_*

Signed-off-by: Owen Smith <owen.smith@citrix.com>
Refactor to improve code clarity

Signed-off-by: Owen Smith <owen.smith@citrix.com>
Add handler for SCSIOP_UNMAP, translating to BLKIF_OP_DISCARD
Note: Storport will not generate SCSIOP_UNMAPs, a filter will be
      required to convert IOCTL_STORAGE_MANAGE_DATA_SET_ATTRIBUTES
	  into SCSIOP_UNMAPs

Signed-off-by: Owen Smith <owen.smith@citrix.com>