vl.c: Print the actual program name in help output

In help() we do what boils down to:

  printf("%s", "qemu");

This seems to be an artifact of be995c27
("removed unused code"), which removed some ifdef'ery that used to print
a different name depending on CONFIG_SOFTMMU.

Instead print the actual program name, originally from argv[0].

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>