libxl: allow dom0 to be destroyed

libxl: allow dom0 to be destroyed

When dom0 is not the hardware domain, it can be destroyed in the same
way as any other service domain.  To avoid accidental use when a domain
is not resolved, destroying domain 0 requires passing -f to xl destroy.
Since the hypervisor already prevents a domain from destroying itself,
this patch is only useful in a disaggregated environment.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Keir Fraser <keir@xen.org>