libxl: introduce flexarray_append_pair

libxl: introduce flexarray_append_pair

Introduce flexarray_append_pair: a simple syntactic sugar to add a pair
of pointers to a flexarray, very useful when adding pairs of strings to
xenstore.

Replace flexarray_vappend calls (and one pair of flexarray_append)
with flexarray_append_pair calls when dealing with flexarrays that are
going to be used with libxl__xs_kvs_of_flexarray:

NULL is a valid pointer value in these cases while flexarray_vappend
uses NULL as vargs terminator, so the old code is buggy.

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