qga/qapi-schema: Document -1 for invalid PCI address fields

The "guest-get-fsinfo" could also be used for non-PCI devices in the
future. And the code in GuestPCIAddress() in qga/commands-win32.c seems
to be using "-1" for fields that it can not determine already. Thus
let's properly document "-1" as value for invalid PCI address fields.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>