memfd_backend_memory_alloc/file_backend_memory_alloc both needlessly
are are calling host_memory_backend_mr_inited() which creates an
illusion that alloc could be called multiple times but it isn't, it's
called once from UserCreatable complete().

Suggested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>