vhost: fix error handling for memory region alloc

callers of vhost_kvzalloc() expect the same behaviour on
allocation error as from kmalloc/vmalloc i.e. NULL return
value. So just return vzmalloc() returned value instead of
returning ERR_PTR(-ENOMEM)

Fixes: 4de7255f ("vhost: extend memory regions allocation to vmalloc")

Spotted-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>