Do not drop kernel cmdline for xen pv domains

Do not drop kernel cmdline for xen pv domains

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