usb: use iovecs in USBPacket

Zap data pointer from USBPacket, add a QEMUIOVector instead.
Add a bunch of helper functions to manage USBPacket data.
Switch over users to the new interface.

Note that USBPacket->len was used for two purposes:  First to
pass in the buffer size and second to return the number of
transfered bytes or the status code on async transfers.  There
is a new result variable for the latter.  A new status code
was added to catch uninitialized result.

Nobody creates iovecs with more than one element (yet).
Some users are (temporarely) limited to iovecs with a single
element to keep the patch size as small as possible.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

Makefile.objs

@@ -172,6 +172,7 @@ user-obj-y += cutils.o cache-utils.o
 hw-obj-y =
 hw-obj-y += vl.o loader.o
 hw-obj-$(CONFIG_VIRTIO) += virtio-console.o
+hw-obj-y += usb-libhw.o
 hw-obj-$(CONFIG_VIRTIO_PCI) += virtio-pci.o
 hw-obj-y += fw_cfg.o
 hw-obj-$(CONFIG_PCI) += pci.o pci_bridge.o