Linux does not support blocks greater than 4 kB anyway, so we might as
well limit blkshift to 12 and thus save us from some potential trouble.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190730114812.10493-1-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Coverity: CID 1403771
Signed-off-by: Max Reitz <mreitz@redhat.com>