Allow xen guests to plug disks of 1 TiB or more

The current xen backend driver implementation uses int64_t variables
to store the size of the corresponding backend disk/file. It also uses
an int64_t variable to store the block size of that image. When writing
the number of sectors (file_size/block_size) to xenstore, however, it
passes these values as 32 bit signed integers. This will cause an
overflow for any disk of 1 TiB or more.

This patch changes the xen backend driver to use a 64 bit integer write
xenstore function.

Signed-off-by: Felipe Franciosi <felipe@paradoxo.org>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

