KVM: x86: make double/triple fault promotion generic to all exceptions

Move Double-Fault generation logic out of page fault
exception generating function to cover more generic case.

Signed-off-by: Eddie Dong <eddie.dong@intel.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>