KVM: reuse memslot in kvm_write_guest_page

Caching memslot value and using mark_page_dirty_in_slot() avoids another
O(log N) search when dirtying the page.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Message-Id: <1428695247-27603-1-git-send-email-rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>