kvmtool/run: append cfg.kernel_cmdline at the end of real_cmdline

This allows the user to always override the paramaters set by lkvm.
Say, currently 'lkvm run -p ro' doesn't work.

To keep the current logic we need to change strstr("root=") to check
cfg.kernel_cmdline, not real_cmdline. And perhaps we can even add a
simple helper add_param(name, val) to make this all more consistent;
it should only append "name=val" to real_cmdline if cfg.kernel_cmdline
doesn't include this paramater.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>