kvmtool: fix build with kernel headers >= 4.12

In Linux 4.12, the header <asm/msr-index.h> has been removed from the
set of headers exported to userspace. Therefore, it cannot be used by
kvmtool anymore. This commit takes the simple approach of duplicating
inside kvmtool the MSR_* definitions that were used from this
<asm/msr-index.h> header.

This fixes:

x86/kvm-cpu.c:7:27: fatal error: asm/msr-index.h: No such file or directory
 #include <asm/msr-index.h>

Which is the second part of:

  http://autobuild.buildroot.net/results/4459a909e735343d1cf768d30466bc3c57eca19e/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>