Next step towards HMM support. For now just silence the retry fault and
optionally redirect the request to the dummy page.

v2: make sure the VM is not destroyed while we handle the fault.
v3: fix VM destroy check, cleanup comments
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>