libxl: Move dm user determination logic into a helper function

libxl: Move dm user determination logic into a helper function

To reliably kill an untrusted devicemodel, we need to know not only
its pid, but its uid.  In preparation for this, move the userid
determination logic into a helper function.

Create a new field, `dm_runas`, in libxl__domain_build_state to store
the value during domain creation.

This change also removes unnecessary duplication of the argument
construction code.

While here, clean up some minor CODING_STYLE infractions (space
between * and variable name).

No functional change intended.

While here, delete some trailing whitespace.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>