libxl: change some structures to unit arrays

libxl: change some structures to unit arrays

In the next patch these variables will turn into actual pointers.

To clarify that patch, prepare the ground by changing these variables
from "struct foo var" to "struct foo var[1]".  This enables accesses
to them and their members to be made as if they were pointers.

No functional change.

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