kvm tools: powerpc: Implement "system-reboot" RTAS call

On some powerpc systems, reboot is implemented by an RTAS call by the
name of "system-reboot". Currently we don't implement it in kvmtool,
which means instead the guest prints an error and spins.

This is particularly annoying because when the guest kernel panics it
will try to reboot, and end up spinning in the guest.

We can't implement reboot properly, ie. causing a reboot, but it's still
preferable to implement it as halt rather than not implementing it at
all.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Pekka Enberg <penberg@kernel.org>