libxl: tidy libxl_get_scheduler() according to CODING_STYLE

libxl: tidy libxl_get_scheduler() according to CODING_STYLE

To more closely follow the guidelines in CODING_STYLE, store the result
of xc_sched_id() in the local variable r, and the check the result of
the call in a separate statement.  Change the type of the output
parameter given to xc_sched_id() from libxl_scheduler to int to match
the libxc interface.

Additionally, change the error log statement to more accurately reflect
the failure.  This is the only functional change introduced by this
patch.

Suggested-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Chester Lin <czylin@uwaterloo.ca>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>