oxenstored: Don't re-open a xenctrl handle for every domain introduction

oxenstored: Don't re-open a xenctrl handle for every domain introduction

Currently, an xc handle is opened in main() which is used for cleanup
activities, and a new xc handle is temporarily opened every time a domain is
introduced.  This is inefficient, and amongst other things, requires full root
privileges for the lifetime of oxenstored.

All code using the Xenctrl handle is in domains.ml, so initialise xc as a
global (now happens just before main() is called) and drop it as a parameter
from Domains.create and Domains.cleanup.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>