Create xmlutils/qemucmdline.py to generate <qemu:commandline> XML

To use get_qemucmdline_xml() function you just need to pass a dict of
{arg: value} as parameter.

Use the created function on vmtemplate.py instead of the plan text to
get the <qemu:commandline> XML for the remote CDROM when libvirt does
not support ISO streaming (but it is supported by QEMU).

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>