bootloader_args is named bootargs in xen-xm

bootloader_args is named bootargs in xen-xm

According to <xen-3.4.3/tools/python/xen/xm/create.py:158>
	gopts.var('bootargs', val='NAME',
			  fn=set_value, default=None,
			  use="Arguments to pass to boot loader")
the "bootloader_args" parameter needs to be translated into "bootargs"
when using "virsh domxml-to-native xen-xm".
The reverse direction (domxml-from-native) is already okay.

This patch fixes domxml-to-native and adds two test files to catch this
problem.

Signed-off-by: Philipp Hahn <hahn@univention.de>