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 <stefanha@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
--
v3:
 * Document qpci_find_capability()
Message-Id: <20191023100425.12168-11-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>