KVM: Fix kvm mmu_notifier initialization order

Like the following, mmu_notifier can be called after registering
immediately. So, kvm have to initialize kvm->mmu_lock before it.

BUG: spinlock bad magic on CPU#0, kswapd0/342
 lock: ffff8800af8c4000, .magic: 00000000, .owner: <none>/-1, .owner_cpu: 0
Pid: 342, comm: kswapd0 Not tainted 2.6.39-rc5+ #1
Call Trace:
 [<ffffffff8118ce61>] spin_bug+0x9c/0xa3
 [<ffffffff8118ce91>] do_raw_spin_lock+0x29/0x13c
 [<ffffffff81024923>] ? flush_tlb_others_ipi+0xaf/0xfd
 [<ffffffff812e22f3>] _raw_spin_lock+0x9/0xb
 [<ffffffffa0582325>] kvm_mmu_notifier_clear_flush_young+0x2c/0x66 [kvm]
 [<ffffffff810d3ff3>] __mmu_notifier_clear_flush_young+0x2b/0x57
 [<ffffffff810c8761>] page_referenced_one+0x88/0xea
 [<ffffffff810c89bf>] page_referenced+0x1fc/0x256
 [<ffffffff810b2771>] shrink_page_list+0x187/0x53a
 [<ffffffff810b2ed7>] shrink_inactive_list+0x1e0/0x33d
 [<ffffffff810acf95>] ? determine_dirtyable_memory+0x15/0x27
 [<ffffffff812e90ee>] ? call_function_single_interrupt+0xe/...