Implement Indirect Operation

Read "feature-max-indirect-segments" and allow BLKIF_OP_INDIRECT operations when enabled.
SRBs will be prepared as BLKIF_OP_INDIRECT when indirect is enabled, and the number of segments would not fit into a single BLKIF_OP_READ or BLKIF_OP_WRITE (i.e. 11 segments). Indirect operations are limited to "feature-max-indirect-segments" segments (currently 256 in upstream blkback) but could contain up-to 4096 segments (grant reference permitting).