xen: automatically create XenBlockDevice-s

xen: automatically create XenBlockDevice-s

This patch adds create and destroy function for XenBlockDevice-s so that
they can be created automatically when the Xen toolstack instantiates a new
PV backend via xenstore. When the XenBlockDevice is created this way it is
also necessary to create a 'drive' which matches the configuration that the
Xen toolstack has written into xenstore. This is done by formulating the
parameters necessary for each 'blockdev' layer of the drive and then using
qmp_blockdev_add() to create the layers. Also, for compatibility with the
legacy 'xen_disk' implementation, an iothread is automatically created for
the new XenBlockDevice. This, like the driver layers, will be destroyed
after the XenBlockDevice is unrealized.

The legacy backend scan for 'qdisk' is removed by this patch, which makes
the 'xen_disk' code is redundant. The code will be removed by a subsequent
patch.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>