PCI: hv: Use zero-length array in struct pci_packet

Use zero-length array in struct pci_packet and rename struct pci_message's
field "message_type" to "type".  This makes the code more readable.

No functionality change.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: KY Srinivasan <kys@microsoft.com>
CC: Jake Oshins <jakeo@microsoft.com>
CC: Haiyang Zhang <haiyangz@microsoft.com>
CC: Vitaly Kuznetsov <vkuznets@redhat.com>