xen: add backend driver support

Impact: backend device support

Add the basic machinery to support backend drivers.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
[corresponds to 79727b851bac in git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git

]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>