xen: defer xenstore initialization until xenstored is started

The xenstore related devices in the kernel cannot be started until
xenstored is running, which will happen later in the Dom0 case. If
start_info_t doesn't contain a valid xenstore event channel, defer all
xenstore related devices attachment to later.

Sponsored by: Citrix Systems R&D

dev/xen/xenstore/xenstore.c:

• Prevent xenstore from trying to attach it's descendant devices if xenstore is not initialized.
• Add a callback in the xenstore interrupt filter that will trigger the plug of xenstore descendant devices on the first received interrupt. This interrupt is generated when xenstored attaches to the event channel, and serves as a notification that xenstored is running.