Implement virtio-blk device

[shengwen-tw]

This pull request aims to provide the support of the virtio-blk device for the project. The modification mainly includes:

1. A code implementation of virtio-blk device
2. Device tree source adjustment
3. Linux kernel configuration adjustment
4. Interrupt assignment / handling of the virtio-blk device

To test the new feature, you need to:

1. Create a test disk image using dd, cfdisk, losetup, and mkfs.ext4, where I followed the guide of the presentation "Embedded linux from scratch in 45 minutes" at page 35.
2. Launch the emulator using:

./semu <linux-image> [<dtb>] [<disk-img>]

The virtio-blk device should be enumerated at /dev/vda during the boot-up process. The following experiment can help to verify the device works properly:

1. Observe the partition table of the disk:

fdisk -l

2. Mount the disk to a directory:

mkdir mnt/
mount /dev/vda1 mnt/

3. Try writing some thing to the mount directory:

echo "Hello World!" > mnt/test.txt

4. Unmount the disk:

umount mnt/

5. After re-launching the semu, the written file should remain existing.

[jserv]

Check kvm-host for generating ext4 file system image for validation.

[jserv]

Update README.md for command line options.

[jserv]

Thank @shengwen-tw for contributing!

[shengwen-tw]

Thanks for the code review!