qga: Rename guest-get-devices return member 'address' to 'id'

Member 'address' is union GuestDeviceAddress with a single branch
GuestDeviceAddressPCI, containing PCI vendor ID and device ID.  This
is not a PCI address.  Type GuestPCIAddress is.  Messed up in recent
commit 2e4211ce "qga: add command guest-get-devices for reporting
VirtIO devices".

Rename type GuestDeviceAddressPCI to GuestDeviceIdPCI, type
GuestDeviceAddress to GuestDeviceId, and member 'address' to 'id'.

Document the member properly while there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>