kvm: Use kvm-kmod headers if available

Since kvm-kmod-2.6.32.2 we have an alternative source for recent KVM
kernel headers. Use it when available and not overruled by --kerneldir.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -1384,7 +1384,7 @@ EOF
             kvm_cflags="$kvm_cflags -I$kerneldir/arch/$cpu/include"
       fi
   else
-      kvm_cflags=""
+    kvm_cflags=`pkg-config --cflags kvm-kmod 2> /dev/null`
   fi
   if compile_prog "$kvm_cflags" "" ; then
     kvm=yes