scripts: Fix premature_failure() check with newer versions of QEMU

QEMU recently changed its output if it could not open a kernel file
from "could not load kernel ..." to "could not open kernel ...", see
QEMU commit 350f5233d755 ("hw/i386/pc: avoid an assignment in if
condition in x86_load_linux()"). Thus we have to adapt our script
that looks for this string accordingly.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>