PCI: hv: Use the correct buffer size in new_pcichild_device()

We don't really need such a big on-stack buffer.  vmbus_sendpacket() here
only uses sizeof(struct pci_child_message).

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jake Oshins <jakeo@microsoft.com>