KVM: MMU: Introduce a helper function for FIFO zapping

Make the code for zapping the oldest mmu page, placed at the tail of the
active list, a separate function.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>