block/raw-posix: Abort on pread beyond end of non-growable file

This shouldn't happen under any normal circumstances. However, it looks like
it's possible to achieve this with corrupted images. Without this patch
raw_pread is hanging in an endless loop in such cases.

The patch is not affecting growable files, for which such reads happen in
normal use cases. raw_pread_aligned already handles these cases and won't
return zero in the first place.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
