qemu-img: Expose PreallocMode for resizing

Add a --preallocation command line option to qemu-img resize which can
be used to set the PreallocMode parameter of blk_truncate().

While touching this code, fix the fact that we did not handle errors
returned by blk_getlength().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170613202107.10125-5-mreitz@redhat.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
