xl: remove all local "ctx" variables

xl: remove all local "ctx" variables

xl has a global "ctx" variable, so there should be no need to pass a
ctx to any function.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
[ ijc -- dropped xl_fork hunks on commit, this aspect was already done in
         xl: track child processes for the benefit of libxl ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>