libsnapshot: Don't try to truncate block devices

When we're writing to a block device, we can't truncate. Ignore those
commands. Truncate mostly just saves time in the read phase by chopping
off unused data in the event finalize wasn't called.

Bug: 172026020
Test: cow_api_test
Change-Id: I3befc71fa9597edf8243d0f9e17440db91409aea