qapi: Define PCIe link speed and width properties

qapi: Define PCIe link speed and width properties

Create properties to be able to define speeds and widths for PCIe
links.  The only tricky bit here is that our get and set callbacks
translate from the fixed QAPI automagic enums to those we define
in PCI code to represent the actual register segment value.

Cc: Eric Blake <eblake@redhat.com>
Tested-by: Geoffrey McRae <geoff@hostfission.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>