Disabling ppc64le hosts for Qemu tests

[Joel Stanley]

Hello Andrew,

I've been trying to get to the bottom of the slow-booting qemu issue
for a few weeks now. We commited a fix that resolve the issue on x86
hosts, but when running the same Qemu build on ppc64le it still has
the "go slow" behaviour.

 https://github.com/openbmc/qemu/issues/14

I propose we run the Romulus Qemu boot test on x86-only build slaves
while we work this out. That will allow us to unblock the kernel
security bumps, which have been pending for a few weeks.

I'll continue working on fixing Qemu in the future, but I don't have
time for the next two weeks due to some higher priority work.

Cheers,

Joel