vmm: Introduce riscv64 architecture support

Integrate all works done previously to enable booting riscv linux on
riscv platforms, example command:

```console
./target/debug/cloud-hypervisor \
        --kernel path/to/kernel \
        --disk path=path/to/disk \
        --cmdline "console=hvc0 root=/dev/vda rw" \
        --cpus boot=1 \
        --memory size=1024M \
        --seccomp false
```

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

vmm/src/pci_segment.rs

@@ -141,7 +141,7 @@ impl PciSegment {
         Ok(segment)
     }
 
-    #[cfg(target_arch = "aarch64")]
+    #[cfg(any(target_arch = "aarch64", target_arch = "riscv64"))]
     pub(crate) fn new_default_segment(
         address_manager: &Arc<AddressManager>,
         mem32_allocator: Arc<Mutex<AddressAllocator>>,