libsnapshot:Snapuserd: IO path support with dm-snapshot target

Integrate Snapuserd daemon with libsnapshot library and handle
IO requests from dm-snap and dm-snap-persistent.

Tested all the 3 COW operations on a compressed
cow-device file generated through vab_converter_payload.

BUG: 163046732
Test: Added test case to independently test snapuserd IO path.
      Manual test on compressed cow file generated and verifying
      md5sum.

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I9d017ac671733a93c3bc21344228260224640c78
[Palmer: clean up some build issues]
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>