pseries: Fix stalls on hypervisor virtual console

pseries: Fix stalls on hypervisor virtual console

A number of users are reporting stalls when using the pseries
hypervisor virtual console.

A simple test case is to paste 15 or 17 characters at a time
into the console. Pasting 15 characters at a time works fine
but pasting 17 characters hangs for a random amount of time.
Other activity (network, qemu monitor etc) unblocks it.

If qemu-char tries to send more than 16 characters at once,
vty_can_receive returns false. At this point we have to
wait for the guest to consume that output. Everything is good
so far.

The problem occurs when the the guest does consume the output.
We need to signal back to the qemu-char layer that we are
ready for more input. Without this we block until something
else kicks us (eg network activity).

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
(cherry picked from commit 7770b6f78a2d655e03852a5de238f5926c92be6a)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>