Update virtio network driver

[Jinhyun]

Hi, all!
 
We removed our RTEMS-virtio driver's dependencies on architecture. We used
built-in function of gcc, __sync_synchronize(). This function generates
proper memory barrier for target architecture on compile time. In addition,
we replaced pcib_conf_read/write functions to pci_read/write_config
functions. We added several lines of codes for exception handling suggested
by Pavel Pisa. Thank you, Pavel Pisa. As we removed dependencies on CPU
architectures, we moved the virtio source directory located at
c/src/lib/libbsp/i386/pc386/ to c/src/lib/libbsp/shared.
 
We plan to enhance the virtio driver for RTEMS, and we have several
additional patches for it. To easily manage this project from our side, we
created Github repository. You can see the latest version of our virtio
driver in our Github: https://github.com/sslab-konkuk/RTEMS-virtio

Recently, we succeeded to test the virtio with Motorola_powerpc BSP. For
this, we made additional modifications. Motorola_powerpc BSP doesn't support
functions such as rtems_insterrupt_handler_install, so we used functions
such as BSP_install_rtems_irq_handler instead of those. Moreover, in order
to make our virtio work on PowerPC, we added initialization codes to
virtio_pci.c because qemu_fakerom, which is the PowerPC bios of
Motorola_powerpc BSP, doesn't initialize the virtio back-end driver. But
some runtime error is occurred on RTEMS master branch, so we will submit
about it again soon.

Best regards,
Jin-Hyun.