[Libguestfs] [PATCH v2v] v2v: -it vddk: Don't use nbdkit readahead filter with VDDK (RHBZ#1832805).

This is the simplest solution to this problem. There are two other possible fixes I considered: Increase the documented limit (see http://libguestfs.org/virt-v2v-input-vmware.1.html#vddk:-esxi-nfc-service...). However at the moment we know the current limit works through extensive testing (without readahead), plus I have no idea nor any way to test if larger limits are supported by all versions of VMware new and old. The limit we are recommending at the moment is the one documented by VMware. Also this would require users to change their VMware settings again, and no doubt introduce confusion for people who have already adjusted them who might not understand that they need to adjust them again for a v2v minor release. Or splitting large requests in nbdkit-vddk-plugin, but it's a bit silly to coalesce requests in a filter and then split them up at a later stage in the pipeline, not to mention error-prone when you consider multithreading etc. Rich.