xen: pass kernel initrd to qemu

xen: pass kernel initrd to qemu

xen side patch to support xen HVM direct kernel boot:
support 'kernel', 'ramdisk', 'cmdline' (and 'root', 'extra' as well
which would be deprecated later) in HVM config file, parse config file,
pass -kernel, -initrd, -append parameters to qemu.

It's working with qemu-xen when using the default BIOS (seabios).

[HVM config example]
name="sles11_sp2"
description="None"
uuid="5c84adcc-bd59-788a-96d2-195f9b599cfe"
memory=512
maxmem=512
vcpus=4
on_poweroff="destroy"
on_reboot="restart"
on_crash="destroy"
localtime=0
keymap="en-us"
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"
disk=[ 'file:/mnt/images/sles11_sp2/disk0.raw,hda,w', ]
vif=[ 'mac=00:16:3e:56:af:69,bridge=br0,type=netfront', ]
stdvga=0
vnc=1
vncunused=1
viridian=0
acpi=1
pae=1
serial="pty"

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