Increase the shutdown timeout to 90 seconds

From Geert Jansen:

"When building images with QEmu using emulation,
I run dangerously close to the shutdown timeout
of 60 seconds. The patch below increases it to
120 seconds."

I modified it to be 90, since a shutdown failure is
not fatal.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>