Use ISTs instead of tasks.

One big remaining difference between 32- and 64-bits is that tasks
on 32-bits take care of saving registers.  ISTs do not, only the
stack is affected.

For now, just save/restore caller-save registers in the assembly
wrapper, since there's just one test that uses ISTs.  Later we
can extend the existing APIs to register exception handlers, so
that they can use tasks or ISTs too.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>