kvmtool/run: set a default cmdline if not set

when starting with custom kernel and disk options, kernel_cmdline is
NULL; it results in a segfault while trying to look for a string
using `strstr`:

__strstr_sse2_unaligned () at ../sysdeps/x86_64/multiarch/strstr-sse2-unaligned.S:40
0x00000000004056bf in kvm_cmd_run_init (argc=<optimized out>, argv=<optimized out>) at builtin-run.c:608
0x000000000040639d in kvm_cmd_run (argc=<optimized out>, argv=<optimized out>, prefix=<optimized out>) at builtin-run.c:659
0x0000000000412b8f in handle_command (command=0x62bbc0 <kvm_commands>, argc=5, argv=0x7fffffffe840) at kvm-cmd.c:84
0x00007ffff7211b45 in __libc_start_main (main=0x403540 <main>, argc=6, argv=0x7fffffffe838, init=<optimized out>, fini=<optimized out>,
  rtld_fini=<optimized out>, stack_end=0x7fffffffe828) at libc-start.c:287
0x0000000000403962 in _start ()

this patch suggests to set a minimal cmdline when kernel_cmdline is NULL

Fixes: 8a7163f3 ("kvmtool/run: append cfg.kernel_cmdline at the end of real_cmdline")
Signed-off-by: William Dauchy <william@gandi.net>
Signed-off-by: Will Deacon <will.deacon@arm.com>