Check for the LOADPARM capabilility and potentially add a loadparm=x to
the "-machine" string for the QEMU command line.

Also add xml2argv test cases for loadparm.
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>