Generic cleanup of the Xen driver code

This is mostly a code alignment fix PR to make the code more readable. I also replaced xen_get_instance and xen_get_xchandle with static inline functions as these are used very heavily all over the code and its a waste to have them as separate functions. Internal calls to xen_get_domainid(vmi) have also been replaced with xen_get_instance(vmi)->domainid to use the inline version.

In addition, the the pause call has been extended to only pause the VM if its not already paused. This is required as starting with Xen 4.4.1 pause/unpause requests are counted by Xen (it's no longer a simple toggle). 
If the domain that we requested doesn't exist. Xen just returns the requested number of domans starting from the specified number. If the start domain doesn't exist, it moves it up to the first one that does.