libxc: save/restore: remove static context variables

libxc: save/restore: remove static context variables

20544:ad9d75d74bd5 and 20545:cc7d66ba0dad seemingly intended to change these
global static variables into stack variables but didn't remove the static
qualifier.

Also zero the entire struct once with memset rather than clearing fields
piecemeal in two different places.

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