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: NFarhan Ali <alifm@linux.vnet.ibm.com>
Reviewed-by: NBjoern Walk <bwalk@linux.vnet.ibm.com>
Reviewed-by: NBoris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Reviewed-by: NMarc Hartmayer <mhartmay@linux.vnet.ibm.com>