Change Domain::GetId so it only throws an exception when an actual error occurs
Instead of blanketly throwing an exception when virDomainGetId returns
-1, check to see if an actual error object exists in virGetLastError; if
it does, then throw the exception as normal, and if it doesn't then just
return null. This will allow the caller to handle a null domain id
without having to clutter up the code with try/catch blocks for
situations that might not be exceptional (i.e., the domain is simply not
running).