vl.c: Avoid segfault when started with no arguments
Fix a bug (introduced in commit a0abe47) where a command line which
specified no machine arguments (either explicitly or implicitly via
-kernel &co) would result in a segfault because of a NULL pointer
returned from qemu_opts_find(qemu_find_opts("machine"), 0).
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>