virttest.virt_vm: Do not check index error by 'self.virtnet < index'.

parameter index may be a string or int. When it is a string,
VMInterfaceIndexError() will be raised. It is not what we want.

No matter string or int used, self.virtnet[index] will raise IndexError()
for unavailable index.

So remove
        if self.virtnet < index:
            raise VMInterfaceIndexError()

And catch
IndexError in graceful_shutdown() in qemu_vm.py.

Signed-off-by: Feng Yang <fyang@redhat.com>