Update QEMU_CMD syntax before qemu upgrade.

The coming upgrade to qemu-kvm 0.15 appears to require a change in the
invocation of qemu in combination with chroot.  This new syntax is
compatible with the current qemu-kvm version as well.

TEST=As follows:
sudo FEATURES="test" emerge-x86-generic -1 verity
sudo FEATURES="test" emerge-arm-generic -1 verity
trybot run of x86-generic-pre-flight-queue

Change-Id: If2c82ea71ca40e5a700ea709f524a87f1a5134a6
Reviewed-on: https://gerrit.chromium.org/gerrit/11131
Reviewed-by: Matt Tennant <mtennant@chromium.org>
Tested-by: Matt Tennant <mtennant@chromium.org>
Commit-Ready: Matt Tennant <mtennant@chromium.org>
1 file changed