Kernel cmdline args can be passed to xen pv domains even when a
bootloader is specified. The current config-to-sxpr mapping
ignores cmdline when bootloader is present.
Since the xend sub-driver is used with many xen toolstack versions,
this patch takes conservative approach of adding an else block to
existing !def->os.bootloader, and only appends sxpr if def->os.cmdline
is non-NULL.
V2: Fix existing testcase broken by this patch and add new testcases
