Migrate virtio-blk from semu

This commit migrates @shengwen-tw's brilliant virtio implementation
from semu, with the following modification:

1. Rename virtio_blk_reg_read/write to virtio_blk_read/write
The original virtio_blk_read verified whether the virtio MMIO was
aligned to 4. Since rv32emu does not enforce alignment checks for UART
and PLIC devices or raise misalignment exceptions, this verification
has been omitted.

2. Implement MMIO_VIRTIOBLK

3. Implement vblk_new() and vblk_delete()
These functions align with the conventions used for UART and PLIC
devices.

4. Introduce new argument '-v' for virtio-blk disk image

README.md

@@ -77,7 +77,7 @@ $ make ENABLE_SYSTEM=1 system
 Build using run using specified images:
 ```shell
 $ make ENABLE_SYSTEM=1
-$ build/rv32emu -k <kernel_img_path> -i <rootfs_img_path>
+$ build/rv32emu -k <kernel_img_path> -i <rootfs_img_path> -v <virtio_img_path>
 ```
 
 #### Build Linux image

src/devices/minimal.dts

@@ -65,5 +65,11 @@
             no-loopback-test;
             clock-frequency = <5000000>; /* the baudrate divisor is ignored */
         };
+
+        blk0: virtio@4200000 {
+            compatible = "virtio,mmio";
+            reg = <0x4200000 0x200>;
+            interrupts = <3>;
+        };
     };
 };