KVM: selftests: Create VM earlier for dirty log test

Since we've just removed the dependency of vm type in previous patch,
now we can create the vm much earlier.  Note that to move it earlier
we used an approximation of number of extra pages but it should be
fine.

This prepares for the follow up patches to finally remove the
duplication of guest mode parsings.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>