docs: Replace every mention of ClearLinux with Ubuntu

Now that our CI has transitioned from ClearLinux to Ubuntu images
exclusively, let's update the documentation to refer to Ubuntu images
instead of ClearLinux's ones.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

docs/vhost-user-blk-testing.md

@@ -73,10 +73,10 @@ VMs run in client mode. They connect to the socket created by the `dpdkvhostuser
 # From the test terminal. We need to create one vhost-user-blk device for the --disk.
 ./cloud-hypervisor \
         --cpus boot=4 \
-        --memory size=1024M,file=/dev/hugepages \
+        --memory size=1024M,hugepages=on \
         --kernel linux/arch/x86/boot/compressed/vmlinux.bin \
-        --cmdline "console=ttyS0 reboot=k panic=1 nomodules i8042.noaux i8042.nomux i8042.nopnp i8042.dumbkbd root=/dev/vda3 iommu=off" \
-        --disk "path=images/clear-kvm.img" "num_queues=4,queue_size=128,vhost_user=true,socket=/var/tmp/vhost.1" \
+        --cmdline "console=ttyS0 root=/dev/vda1 rw iommu=off" \
+        --disk path=images/focal-server-cloudimg-amd64.raw vhost_user=true,socket=/var/tmp/vhost.1,num_queues=4,queue_size=128 \
         --console off \
         --serial tty \
         --rng
@@ -88,11 +88,11 @@ login in guest
 
 # Use lsblk command to find out vhost-user-blk device
 lsblk
-NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
-vda    253:0    0  8.5G  0 disk
-├─vda1 253:1    0  511M  0 part
-├─vda2 253:2    0   32M  0 part [SWAP]
-└─vda3 253:3    0    8G  0 part /
+NAME    MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
+vda     252:0    0  2.2G  0 disk 
+├─vda1  252:1    0  2.1G  0 part /
+├─vda14 252:14   0    4M  0 part 
+└─vda15 252:15   0  106M  0 part /boot/efi
 vdb    253:16   0  512M  0 disk
 
 The vhost-user-blk device is /dev/vdb