KVM: MMU: filter out the mmio pfn from the fault pfn

KVM: MMU: filter out the mmio pfn from the fault pfn

If the page fault is caused by mmio, the gfn can not be found in memslots, and
'bad_pfn' is returned on gfn_to_hva path, so we can use 'bad_pfn' to identify
the mmio page fault.
And, to clarify the meaning of mmio pfn, we return fault page instead of bad
page when the gfn is not allowd to prefetch

Signed-off-by: Xiao Guangrong <[email protected]>
Signed-off-by: Avi Kivity <[email protected]>