hostmem-ram: don't exit qemu if size of memory-backend-ram is way too big

When using monitor command object_add to add a memory backend whose
size is way too big to allocate memory for it, qemu just exits. In
the case we'd better give an error message and keep guest running.

The problem can be reproduced as follows:

1. run qemu
2. (monitor)object_add memory-backend-ram,size=100000G,id=ram0
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NHu Tao <hutao@cn.fujitsu.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

backends/hostmem-ram.c

@@ -27,7 +27,7 @@ ram_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
 
     path = object_get_canonical_path_component(OBJECT(backend));
     memory_region_init_ram(&backend->mr, OBJECT(backend), path,
-                           backend->size, &error_abort);
+                           backend->size, errp);
     g_free(path);
 }