maint: fix comma style issues: xen

maint: fix comma style issues: xen

Most of our code base uses space after comma but not before;
fix the remaining uses before adding a syntax check.

* src/libxl/libxl_driver.c: Consistently use commas.
* src/xen/xend_internal.c: Likewise.
* src/xen/xs_internal.c: Likewise.
* src/xenapi/xenapi_driver.c: Likewise.
* src/xenapi/xenapi_utils.c: Likewise.
* src/xenxs/xen_sxpr.c: Likewise.
* src/xenxs/xen_xm.c: Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>