engines/sg: support trim operations via the UNMAP command

engines/sg: support trim operations via the UNMAP command

Multiple trim ranges can be submitted with a single UNMAP command
for character devices (e.g., /dev/sg1) using iodepth_batch. For
example, with iodepth_batch=4, each UNMAP command will include
four ranges. IOPS will still be reported as if four IO operations
were carried out.

Note that currently sg only supports a maximum of 16 commands
in flight per file. So be sure to maintain (iodepth/iodepth_batch) <= 16.