xen: import a proper event channel user-space device

The user-space event channel device is used by applications to receive
and send event channel interrupts. This device is based on the Linux
evtchn device.

Sponsored by: Citrix Systems R&D

xen/evtchn/evtchn_dev.c:

• Remove the old event channel device, which was already disabled in the build system.

dev/xen/evtchn/evtchn_dev.c:

• Import a new event channel device based on the one present in Linux.
• This device allows the following operations:
  • Bind VIRQ event channels (ioctl).
  • Bind regular event channels (ioctl).
  • Create and bind new event channels (ioctl).
  • Unbind event channels (ioctl).
  • Send notifications to event channels (ioctl).
  • Reset the device shared memory ring (ioctl).
  • Unmask event channels (write).
  • Receive event channel upcalls (read).
• The new code is MP safe, and can be used concurrently.

conf/files:

• Add the new device to the build system.