xl.cfg: add 'cmdline' in config file

xl.cfg: add 'cmdline' in config file

Currently in xl.cfg, use 'root' and 'extra' to generate the command
line. 'cmdline' could be a more generic equivalent. So, add 'cmdline'
in xl.cfg and let it be preferred. 'root' and 'extra' still works.
But when 'cmdline' is specified, 'root' and 'extra' will be ignored.

[HVM config example]
[snip]
builder="hvm"
device_model_override="/home/cyliu/git/qemu/x86_64-softmmu/qemu-system-x86_64"
kernel="/mnt/vmlinuz-3.0.13-0.27-default"
ramdisk="/mnt/initrd-3.0.13-0.27-default"
root="/dev/hda2"
extra="console=tty0 console=ttyS0"
[snip]

or:

[snip]
builder="hvm"
device_model_override="/home/cyliu/git/qemu/x86_64-softmmu/qemu-system-x86_64"
kernel="/mnt/vmlinuz-3.0.13-0.27-default"
ramdisk="/mnt/initrd-3.0.13-0.27-default"
cmdline="root=/dev/hda2 console=tty0 console=ttyS0"
[snip]

Signed-off-by: Chunyan Liu <cyliu@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>