Upsteam commit 9fa06385f6 ("kvm: Mark full address range dirty on live
migration start") requests dirty logging on the entire 64-bit address
space.  This triggered an allocation in qemu-kvm.c for this range,
which oomed.

Fix by allocating the buffer when we know the actual slot, thus
allocating exactly as much memory as necessary.

Signed-off-by: Avi Kivity <avi@redhat.com>