qemu/x86*: update instructions to use virtio

Switch the invocation command to use virtio-blk and virtio-net for
better performance.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/qemu/x86/readme.txt

@@ -1,9 +1,9 @@
 Run the emulation with:
 
-  qemu-system-i386 -M pc -kernel output/images/bzImage -drive file=output/images/rootfs.ext2,if=ide,format=raw -append root=/dev/sda -net nic,model=rtl8139 -net user
+  qemu-system-i386 -M pc -kernel output/images/bzImage -drive file=output/images/rootfs.ext2,if=virtio,format=raw -append root=/dev/vda -net nic,model=virtio -net user
 
 Optionally add -smp N to emulate a SMP system with N CPUs.
 
 The login prompt will appear in the graphical window.
 
-Tested with QEMU 2.3.0
+Tested with QEMU 2.5.0

board/qemu/x86_64/readme.txt

@@ -1,9 +1,9 @@
 Run the emulation with:
 
-  qemu-system-x86_64 -M pc -kernel output/images/bzImage -drive file=output/images/rootfs.ext2,if=ide,format=raw -append root=/dev/sda -net nic,model=rtl8139 -net user
+  qemu-system-x86_64 -M pc -kernel output/images/bzImage -drive file=output/images/rootfs.ext2,if=virtio,format=raw -append root=/dev/vda -net nic,model=virtio -net user
 
 Optionally add -smp N to emulate a SMP system with N CPUs.
 
 The login prompt will appear in the graphical window.
 
-Tested with QEMU 2.3.0
+Tested with QEMU 2.5.0