usb: Introduce Xen pvUSB frontend (xen hcd)

Introduces the Xen pvUSB frontend. With pvUSB it is possible for a Xen
domU to communicate with a USB device assigned to that domU. The
communication is all done via the pvUSB backend in a driver domain
(usually Dom0) which is owner of the physical device.

The pvUSB frontend is a USB hcd for a virtual USB host connector.

The code is taken from the pvUSB implementation in Xen done by Fujitsu
based on Linux kernel 2.6.18.

Changes from the original version are:
- port to upstream kernel
- put all code in just one source file
- move module to appropriate location in kernel tree
- adapt to Linux style guide
- minor code modifications to increase readability

Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20211123132048.5335-3-jgross@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/Kconfig

@@ -772,3 +772,14 @@ config USB_HCD_TEST_MODE
 	  This option is of interest only to developers who need to validate
 	  their USB hardware designs.  It is not needed for normal use.  If
 	  unsure, say N.
+
+config USB_XEN_HCD
+	tristate "Xen usb virtual host driver"
+	depends on XEN
+	select XEN_XENBUS_FRONTEND
+	help
+	  The Xen usb virtual host driver serves as a frontend driver enabling
+	  a Xen guest system to access USB Devices passed through to the guest
+	  by the Xen host (usually Dom0).
+	  Only needed if the kernel is running in a Xen guest and generic
+	  access to a USB device is needed.

drivers/usb/host/Makefile

@@ -85,3 +85,4 @@ obj-$(CONFIG_USB_HCD_BCMA)	+= bcma-hcd.o
 obj-$(CONFIG_USB_HCD_SSB)	+= ssb-hcd.o
 obj-$(CONFIG_USB_FOTG210_HCD)	+= fotg210-hcd.o
 obj-$(CONFIG_USB_MAX3421_HCD)	+= max3421-hcd.o
+obj-$(CONFIG_USB_XEN_HCD)	+= xen-hcd.o