qcow2: Use pread for inactive L1 in overlap check

Currently, qcow2_check_metadata_overlap uses bdrv_read to read inactive
L1 tables from disk. The number of sectors to read is calculated through
a truncating integer division, therefore, if the L1 table size is not a
multiple of the sector size, the final entries will not be read and
their entries in memory remain undefined (from the g_malloc).
Using bdrv_pread fixes this.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
