Introduce a Xen xl parser

This parser allows for users to convert the new xl disk format and
spice graphics config to libvirt xml format and vice versa. Regarding
the spice graphics config, the code is pretty much straight forward.
For the disk {formating, parsing}, this parser takes care of the new
xl format which include positional parameters and key/value parameters.
In xl format disk config a <diskspec> consists of parameters separated by
commas. If the parameters do not contain an '=' they are automatically
assigned to certain options following the order below

   target, format, vdev, access

The above are the only mandatory parameters in the <diskspec> but there
are many more disk config options. These options can be specified as
key=value pairs. This takes care of the rest of the options such as

  devtype, backend, backendtype, script, direct-io-safe,

The positional paramters can also be specified in key/value form
for example

    /dev/vg/guest-volume,,hda
    /dev/vg/guest-volume,raw,hda,rw
    format=raw, vdev=hda, access=rw, target=/dev/vg/guest-volume

are interpleted to one config.

In xm format, the above diskspec would be written as

phy:/dev/vg/guest-volume,hda,w

The disk parser is based on the same parser used successfully by
the Xen project for several years now.  Ian Jackson authored the
scanner, which is used by this commit with mimimal changes.  Only
the PREFIX option is changed, to produce function and file names
more consistent with libvirt's convention.
Signed-off-by: NKiarie Kahurani <davidkiarie4@gmail.com>
Signed-off-by: NJim Fehlig <jfehlig@suse.com>