libqos: add iteration support to qpci_find_capability()

libqos: add iteration support to qpci_find_capability()

VIRTIO 1.0 PCI devices have multiple PCI_CAP_ID_VNDR capabilities so we
need a way to iterate over them.  Extend qpci_find_capability() to take
the last address.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
--
v3:
 * Document qpci_find_capability()
Message-Id: <20191023100425[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>