KVM: introduce readonly memslot

Changelog:
- fix endless retrying for unhandleable instruction which accesses on readonly
  host memory
- divide slot->flags by 16:16, the lower part is visible for userspace, the
  reset is internally used in kvm, and document this in the code
- check slot->flags for gfn_to_hva_memslot

The test case can be found at:
http://lkml.indiana.edu/hypermail/linux/kernel/1207.2/008...

In current code, if we map a readonly memory space from host to guest
and the page is not currently mapped in the host, we will get a fault-pfn
and async is not allowed, then the vm will crash.

As Avi's suggestion, We introduce readonly memory region to map ROM/ROMD
to the guest, read access is happy for readonly memslot, write access on
readonly memslot will cause KVM_EXIT_MMIO exit.

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