kvm tools: Enable fast string operations

Recent kernels check for MSR_IA32_MISC_ENABLE_FAST_STRING in the
MSR_IA32_MISC_ENABLE MSR before enabling reps/movs memcpy.

So far we didn't set it, and got a slower memcpy and a warning:

	[    0.000000] Disabled fast string operations

This patch enables fast string operations.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>