KVM: selftests: Fix build with "make ARCH=x86_64"

Marcelo reports that kvm selftests fail to build with
"make ARCH=x86_64":

gcc -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99
 -fno-stack-protector -fno-PIE -I../../../../tools/include
 -I../../../../tools/arch/x86_64/include  -I../../../../usr/include/
 -Iinclude -Ilib -Iinclude/x86_64 -I.. -c lib/kvm_util.c
 -o /var/tmp/20200604202744-bin/lib/kvm_util.o

In file included from lib/kvm_util.c:11:
include/x86_64/processor.h:14:10: fatal error: asm/msr-index.h: No such
 file or directory

 #include <asm/msr-index.h>
          ^~~~~~~~~~~~~~~~~
compilation terminated.

"make ARCH=x86", however, works. The problem is that arch specific headers
for x86_64 live in 'tools/arch/x86/include', not in
'tools/arch/x86_64/include'.

Fixes: 66d69e08 ("selftests: fix kvm relocatable native/cross builds and installs")
Reported-by: Marcelo Bandeira Condotta <mcondotta@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200605142028.550068-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>