KVM: MMU: fast page fault

Thanks for Avi and Marcelo's review, i have simplified the whole things
in this version:
- it only fix the page fault with PFEC.P = 1 && PFEC.W = 0 that means
  unlock set_spte path can be dropped.

- it only fixes the page fault caused by dirty-log

In this version, all the information we need is from spte, the
SPTE_ALLOW_WRITE bit and SPTE_WRITE_PROTECT bit:
   - SPTE_ALLOW_WRITE is set if the gpte is writable and the pfn pointed
     by the spte is writable on host.
   - SPTE_WRITE_PROTECT is set if the spte is write-protected by shadow
     page table protection.

All these bits can be protected by cmpxchg, now, all the things is fairly
simple than before. :)

Performance test:

autotest migration:
(Host: Intel(R) Xeon(R) CPU           X5690  @ 3.47GHz * 12 + 32G)

- For ept:

Before:
                    smp2.Fedora.16.64.migrate
Times   .unix      .with_autotest.dbench.unix     total
 1       104           214                         323
 2       68            238                         310
 3       68            242                         314

After:
                    smp2.Fedora.16.64.migrate
Times   .unix      .with_autotest.dbench.unix     total
 1       101           190                         295
 2       67            188                         259
 3       66            217                         289


- For shadow mmu:

Before:
                    smp2.Fedora.16.64.migrate
Times   .unix      .with_autotest.dbench.unix     total
 1       103           235                         342
 2       64            219                         286
 3       68            234                         305

After:
                    smp2.Fedora.16.64.migrate
Times   .unix      .with_autotest.dbench.unix     total
 1       104           220                         328
 2       65            204                         273
 3       64            219                         286

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