qxl: replace pipe signaling with bottom half

qxl: replace pipe signaling with bottom half

qxl creates a pipe, then writes something to it to wake up the iothread
from the spice server thread to raise an irq.  These days qemu bottom
halves can be scheduled from threads and signals, so there is no reason
to do this any more.  Time to clean it up.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>