kvm: Fix warning from static code analysis

Report from smatch:

kvm-all.c:1373 kvm_init(135) warn:
 variable dereferenced before check 's' (see line 1360)

's' cannot by NULL (it was alloced using g_malloc0), so there is no need
to check it here.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
