kvm tools: do not append root=/dev/vda if passed via command line

kvm tools: do not append root=/dev/vda if passed via command line

Hardcoding the root= option prevents an existing qemu-kvm based disk
image using LVM from booting. It fails to find the root filesystem.
By making the root parameter optional if given on the command line, the
image boots correctly. For example,

  ./kvm --kernel=/tmp/vmlinuz-2.6.38 \
        --initrd=/tmp/initramfs-2.6.38.img \
        --image=/home/dsa/vm/images/f14/nkvm-f14.img \
        --mem=1024
        --params="ro root=/dev/mapper/vg_f14vm-lv_root"

works now.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>