KVM: Fix IOMMU memslot reference warning

This patch fixes the following warning.

===================================================
[ INFO: suspicious rcu_dereference_check() usage. ]
---------------------------------------------------
include/linux/kvm_host.h:259 invoked rcu_dereference_check() without
protection!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 0
no locks held by qemu-system-x86/29679.

stack backtrace:
Pid: 29679, comm: qemu-system-x86 Not tainted 2.6.35-rc3+ #200
Call Trace:
 [<ffffffff810a224e>] lockdep_rcu_dereference+0xa8/0xb1
 [<ffffffffa018a06f>] kvm_iommu_unmap_memslots+0xc9/0xde [kvm]
 [<ffffffffa018a0c4>] kvm_iommu_unmap_guest+0x40/0x4e [kvm]
 [<ffffffffa018f772>] kvm_arch_destroy_vm+0x1a/0x186 [kvm]
 [<ffffffffa01800d0>] kvm_put_kvm+0x110/0x167 [kvm]
 [<ffffffffa0180ecc>] kvm_vcpu_release+0x18/0x1c [kvm]
 [<ffffffff81156f5d>] fput+0x22a/0x3a0
 [<ffffffff81152288>] filp_close+0xb4/0xcd
 [<ffffffff8106599f>] put_files_struct+0x1b7/0x36b
 [<ffffffff81065830>] ? put_files_struct+0x48/0x36b
 [<ffffffff8131ee59>] ? do_raw_spin_unlock+0x118/0x160
 [<ffffffff81065bc0>] exit_files+0x6d/0x75
 [<ffffffff81068348>] do_exit+0x47d/0xc60
 [<ffffffff8177e7b5>] ? _raw_spin_unlock_irq+0x30/0x36
 [<ffffffff81068bfa>] do_group_exit+0xcf/0x134
 [<ffffffff81080790>] get_signal_to_deliver+0x732/0x81d
 [<ffffffff81095996>] ? cpu_clock+0x4e/0x60
 [<ffffffff81002082>] do_notify_resume+0x117/0xc43
 [<ffffffff810a2fa3>] ? trace_hardirqs_on+0xd/0xf
 [<ffffffff81080d79>] ? sys_rt_sigtimedwait+0x2b5/0x3bf
 [<ffffffff8177d9f2>] ? trace_hardirqs_off_thunk+0x3a/0x3c
 [<ffffffff81003221>] ? sysret_signal+0x5/0x3d
 [<ffffffff8100343b>] int_signal+0x12/0x17

Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>