Direct Linux boot: XenD changes for HVM direct Linux boot.

Direct Linux boot: XenD changes for HVM direct Linux boot.

This patch provides the tools support for direct kernel boot of
HVM guests. Currently the config files in /etc/xen support the
args  'kernel', 'ramdisk' and 'extra'. For PV guests these have
the obvious meaning.

Unfortunately HVM guest configs hijacked the 'kernel' parameter
and use it to refer to the path of the HVM firmware. So, this
patch adds a new config file parameter called 'loader' which is
used to refer to the HVM firmware instead.

The conventions for loading the initrd image say that it should
live at the end of memory. This requires QEMU to know the size
of the guest's initial RAM allocation, so image.py is changed
to pass the '-m' flag to QEMU.

The HVMImageHandler class in image.py is changed so that if the
'kernel', 'ramdisk' or 'extra' params were given in the config
these are passed to QEMU with the '-kernel', '-initrd' and
'-append' flags respectively.

Finally, the 'loader' param is used as the arg to 'xc_hvm_build'
instead of the old 'kernel' param.

For the sake of compatability with old HVM guest config files,
if the config file has a 'kernel' param whose path matches that
of the HVM firmware, then we automatically convert this 'kernel'
param into the 'loader' param. This ensures existing HVM guests
work without changes required.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>