qcow2: Separate qcow2_check_read_snapshot_table()

qcow2: Separate qcow2_check_read_snapshot_table()

Reading the snapshot table can fail.  That is a problem when we want to
repair the image.

Therefore, stop reading the snapshot table in qcow2_do_open() in check
mode.  Instead, add a new function qcow2_check_read_snapshot_table()
that reads the snapshot table at a later point.  In the future, we want
to handle errors here and fix them.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20191011152814.14791-9-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>