conf: add rawio attribute to disk element of domain XML

conf: add rawio attribute to disk element of domain XML

 This patch adds a new attribute "rawio" to the "disk" element
 of domain XML. Valid values of "rawio" attribute are "yes"
 and "no".
 rawio='yes' indicates the disk is desirous of CAP_SYS_RAWIO.

 If you specify the following XML:

 <disk type='block' device='lun' rawio='yes'>
  ...
 </disk>

 the domain will be granted CAP_SYS_RAWIO.
 (of course, the domain have to be executed with root privilege)

NOTE:
   - "rawio" attribute is only valid when device='lun'
   - At the moment, any other disks you won't use rawio can use rawio.

Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>