kvm: move kvm_set_phys_mem around

move kvm_set_phys_mem so that it will
be later available earlier in the file.
needed for next patch using memory notifiers.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>