tests: Fix 'valid_hosts' field for KVM tests

In some cases where we wish to use X86 KVM the 'valid_hosts' field was
set to 'constants.supported_hosts'. This can cause problems as the only
valid host for an X86 KVM test is X86. This patch fixes this.

This fix will likely fix some flakey errors observed in the Nightly
tests.

Change-Id: Id0a906ca6602c0358ae632cce314769807bf6e07
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58970
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
4 files changed