xen: fix usage of kern_getenv in PVH code

The value returned by kern_getenv should be freed using freeenv.

Reported by:	Coverity
CID:		1248852
Sponsored by: Citrix Systems R&D
xen: allow to register event channels without handlers

This is needed by the event channel user-space device, that requires
registering event channels without unmasking them. intr_add_handler
will unconditionally unmask the event channel, so we avoid calling it
if no filter/handler is provided, and then the user will be in charge
of calling it when ready.

In order to do this, we need to change the opaque type
xen_intr_handle_t to contain the event channel port instead of the
opaque cookie returned by intr_add_handler, since now registration of
event channels without handlers are allowed. The cookie will now be
stored inside of the private xenisrc struct. Also, introduce a new
function called xen_intr_add_handler that allows adding a
filter/handler after the event channel has been registered.

Sponsored by: Citrix Systems R&D

x86/xen/xen_intr.c:
 - Leave the event channel without a handler if no filter/handler is
   provided to xen_intr_bind_isrc.
 - Don't perform an evtchn_mask_port, intr_add_handler will already do
   it.
 - Change the opaque type xen_intr_handle_t to contain a pointer to
   the event channel port number, and make the necessary changes to
   related functions.
 - Introduce a new function called xen_intr_add_handler that can be
   used to add filter/handlers to an event channel after registration.

xen/xen_intr.h:
 - Add prototype of xen_intr_add_handler.
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.
xen: add a Xen to BSD error translation

Since Xen and FreeBSD error codes are completely different add a
translation layer in order to convert Xen error codes into native
FreeBSD error codes. This will be used by the privcmd device, which
needs to return the hypercall errors into user-space.

Sponsored by: Citrix Systems R&D

xen/error.h:
 - Import Xen error codes.
 - Create a table to map Xen error codes into FreeBSD native error
   codes.
 - Create an inline function that performs the translation.