migration/block: replace uses of blk_nb_sectors that do not check result

Uses of blk_nb_sectors must check whether the result is negative.
Otherwise, underflow can happen.  Fortunately, alloc_aio_bitmap()
and bmds_aio_inflight() both have an alternative way to retrieve the
number of sectors in the file.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20230407153303.391121-6-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>