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.

BUG=chromium-os:22309
TEST=As follows:
sudo FEATURES="test" emerge-x86-generic -1 cros_boot_mode
sudo FEATURES="test" emerge-arm-generic -1 cros_boot_mode
trybot run of x86-generic-pre-flight-queue

Change-Id: I0d3297bd302bad24e84d7c4b61e2397b4a74d1fb
Reviewed-on: https://gerrit.chromium.org/gerrit/11132
Reviewed-by: Matt Tennant <mtennant@chromium.org>
Tested-by: Matt Tennant <mtennant@chromium.org>
Commit-Ready: Matt Tennant <mtennant@chromium.org>
1 file changed