gpiodev: Add gpiobackend over GUSE

gpiodev: Add gpiobackend over GUSE

Add GUSE (FUSE based kernel module similiar to CUSE) based backend.

This allows transparent usage of Linux GPIO UAPI based tools like
in kernel tools/gpio or libgpiod.

libgpiod requires some modification to allow "/sys/class/guse" in
gpiod_check_gpiochip_device().

It requires guse module to be loaded and providing DEVICE()->id
for GPIO module, for example:

```
DEVICE(&s->gpio)->id = g_strdup("aspeed-gpio0");
```

The id should be provided to gpiodev with any `devname` that doesn't
exists in /dev:

```
-gpiodev guse,id=aspeed-gpio0,devname=gpiochip10
```

That /dev/gpiochip10 can be used in the same way we usually operate with
gpiochip's.

Link: http://git.maquefel.me/?p=qemu-gpiodev/libgpiod.git;a=shortlog;h=refs/heads/nshubin/guse-fix
Link: http://git.maquefel.me/?p=qemu-gpiodev/guse.git;a=summary
Link: http://git.maquefel.me/?p=qemu-gpiodev/libfuse.git;a=shortlog;h=refs/heads/nshubin/guse
Signed-off-by: Nikita Shubin <n.shubin@yadro.com>