hw/misc: don't create pvpanic device by default

hw/misc: don't create pvpanic device by default

This patch is based on Hu Tao's:
http://lists.nongnu.org/archive/html/qemu-devel/2013-08/msg00124.html

No need to hard-code pvpanic as part of the machine.
It can be added with "-device pvpanic" from command line (The next patch).
Anyway, for backport compatibility it is still part of 1.5
machine.

Signed-off-by: Marcel Apfelbaum <[email protected]>
Reviewed-by: Hu Tao <[email protected]>
Message-id: 1376233843[email protected]
Signed-off-by: Anthony Liguori <[email protected]>