kvm tools: Fix required boot protocol version

Cyrill has an old bzImage that SIGSEGVs when running under KVM because
'cmdline_size' is bogus. Reading Documentation/x86/boot.txt, it turns out
32-bit 'cmdline_size' requires boot protocol 2.06 or later so fix up
BOOT_PROTOCOL_REQUIRED.

Reported-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>