xen/blkfront: use the correct type for disk sectors

The correct type to use to represent disk sectors is blkif_sector_t
(which is an uint64_t underneath). This avoid truncation of the disk
size calculation when resizing on i386, as otherwise the calculation
of d_mediasize in xbd_connect is truncated to the size of unsigned
long, which is 32bits on i386.

Note this issue didn't affect amd64, because the size of unsigned long
is 64bits there.

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