libsnapshot: Fetch device size from header

Now that V3 is enabled, relax the header version check.
For V3, header op_count_max contains the information of the device size.

Bug: 299011882
Test: snapshotctl map-snapshots on Pixel with V3 format

Change-Id: Ia1cb20b24857136a742e20408ee95e56e98b256a
Signed-off-by: Akilesh Kailash <akailash@google.com>