libxl: add LIBXL_HAVE_<foo> for outstanding_pages and outstanding_memkb

libxl: add LIBXL_HAVE_<foo> for outstanding_pages and outstanding_memkb

Commits d0782481  ("xl: export 'outstanding_pages' value from xcinfo")
and bec8f17e ("xen: Remove the XENMEM_get_oustanding_pages and provide
the data via xc_phys_info") added these two fields in libxl_physinfo
and in libxl_dominfo, respectively, but did not include the needed
LIBXL_HAVE_<foo> runes. Adding them.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>