PCI: hv: Handle vmbus_sendpacket() failure in hv_compose_msi_msg()

Handle vmbus_sendpacket() failure in hv_compose_msi_msg().

I happened to find this when reading the code.  I didn't get a real issue
however.

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>