Limit reliance on Xenstore in VMI_XEN mode

This branch builds on top of the custom-init and invalid-domid branches.

When performing Dom0 disaggregation, Xenstore requires additional privileges to be set for each domain to be accessed by a secondary domain running LibVMI. The use of Xenstore in LibVMI is currently tied to two pieces of information: determining a domains ID from its name, and getting the size of the memory of the domain. While the domain's name is a piece of information that can only be accessed through Xenstore, the available memory can be obtained from other sources (such as the getdomaininfo hypercall already noted in the comments of the function xen_get_memsize). When running LibVMI in a domain other then Dom0 limiting reliance on Xenstore is required so that LibVMI can initialize provided only with a domain ID.