parallels: reject a bitmap L1 entry outside the data area

Nothing checks where a bitmap L1 entry points. The entry is turned into
an offset and the cluster is read from there, so the image decides which
part of the file, if any, is deserialized as bitmap data.

A short read on the protocol node is zero filled rather than refused, so
an entry beyond the end of the file does not even fail: the bitmap
quietly loads as completely clean. An entry below data_off deserializes
the header and the BAT as bitmap data instead. Both cases used to be
harmless in the sense that the extension was only parsed for read-only
images, but the bitmap becomes writable and is stored back as
authoritative once the image can be opened read-write.

Bound the entry the way parallels_check_outside_image() bounds a BAT
entry: it has to address the data area of the image file. The offset
computation is bounded first, as the entry is a 64 bit value coming from
the image and the shift by BDRV_SECTOR_BITS would overflow.

The overflow case was reported as an I/O error before, so the test
expectation changes along with it.

Fixes: baefd977002e ("parallels: support bitmap extension for read-only mode")
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>