pseries: Check we have a chardev in spapr_vty_init()

If qemu is run like:

 qemu-system-ppc64 -nodefaults -device spapr-vty

We end up in spapr_vty_init() with dev->chardev == NULL. Currently
that leads to a segfault because we unconditionally call
qemu_chr_add_handlers().

Although we could make that call conditional, I think a spapr-vty
without a chardev is basically useless so fail the init. This is
similar to what the serial code does for example.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
