KVM: let the error pfn not depend on error code

Currently, we use the error code as error pfn to indicat the error
condition, it is not straightforward and it will not work on PAE
32-bit cpu with huge memory, since the valid physical address
can be at most 52 bits

For the normal pfn, the highest 12 bits should be zero, so we can
mask these bits to indicate the error.

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>