Enable indirect segment I/Os by default in the Xen blkfront driver when
running on EC2. Due to improvements in EC2, the performance penalty which
was present on some EC2 instances no longer exists, and enabling this
feature now consistently yields ~20% higher throughput with equal or lower
latency.
Reverts: r286063
Approved by: re (gjb)
MFC after: 2 weeks
Relnotes: Improved disk throughput on EC2