The most formal form of xl disk configuration uses key=value
syntax to define each configuration item, e.g.

format=raw, vdev=xvda, access=rw, backendtype=phy, target=disksrc

Change the xl disk formatter to produce this syntax, which allows
target= to contain meta info needed to setup a network-based
disksrc (e.g. rbd, nbd, iscsi). For details on xl disk config
format, see  $xen-src/docs/misc/xl-disk-configuration.txt

Update the disk config in the tests to use the formal syntax.
But add tests to ensure disks specified with the positional
parameter syntax are correctly converted to <disk> XML.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>