xl/libxl: add a blkdev_start parameter

xl/libxl: add a blkdev_start parameter

Introduce a blkdev_start in xl.conf and a corresponding string in
libxl_domain_build_info.

Add a blkdev_start parameter to libxl__device_disk_local_attach: it is
going to be used in a following patch.

blkdev_start specifies the first block device to be used for temporary
block device allocations by the toolstack.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>