shadow: Don't invoke a shutdown on reboot unless in a reboot loop

shadow: Don't invoke a shutdown on reboot unless in a reboot loop

Old versions of KVM would map the same writable copy of the BIOS at
both 0x000f0000 and 0xffff0000.  As a result, a reboot on these
machines would result in a reboot loop.  So, the code attempts to
check for that situation and invoke a shutdown instead.

Commit b837e68d changed the check to run prior to the first reboot.
However, this broke reboots on the QEMU isapc machine type.  Change
the reboot loop check to only be invoked after at least one reboot has
been attempted.

Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>