kvm/valgrind: don't mark memory as initialized

since commit 7dda5dc8 ("migration: initialize RAM to zero") the
guest memory is defined zero. No need to call valgrind on guest memory.
This reverts commit 62fe8331 ("qemu: Use valgrind annotations to
mark kvm guest memory as defined") thus speeding up kvm start if
<includedir>/valgrind/valgrind.h is available.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>