conf: Add loadparm boot option for a boot device

conf: Add loadparm boot option for a boot device

Update the per device boot schema to add an optional loadparm parameter.

eg: <boot order='1' loadparm='2'/>

Extend the virDomainDeviceInfo to support loadparm option.
Modify the appropriate functions to parse loadparm from boot device xml.
Add the xml2xml test to validate the field.

Signed-off-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>