* src/xen/xen_hypervisor.c: Remove all "domain == NULL" tests.
* src/xen/xen_hypervisor.h: Instead, use ATTRIBUTE_NONNULL to
mark each "domain" parameter as "known always to be non-NULL".
The xenstore database sometimes has stale domain IDs which are not
present in the hypervisor anymore. Filter these out to avoid causing
confusion
* src/xen/xs_internal.c: Filter domain IDs against HV's list
* src/xen/xen_hypervisor.h, src/xen/xen_hypervisor.c: Add new
xenHypervisorHasDomain() method for checking ID validity