cros_vm: use the SDK's version of qemu

By default, use the version of qemu shipped with the SDK rather than the
default one shipped with the system that may not support all required features
(e.g. support for virtio_gpu).
This also gives us a more consistent behaviour when VMs are deployed on
different hosts with different versions of qemu.
Requires CL:351861 so qemu can be used from outside the chroot.

BUG=chromium:618431
BUG=chromium:636309
TEST=cbuildbot --remote cyan-android-pfq after applying CL:389132 to
enable VM tests.
TEST=cros_run_vm_update --qemu_binary='xxxx' and check that the correct
qemu binary gets picked.
CQ-DEPEND=CL:351861

Change-Id: Ibd7e120229b0a64b3d38a7cb779d5efbdd8914bd
Reviewed-on: https://chromium-review.googlesource.com/389152
Commit-Ready: Nicolas Norvez <norvez@chromium.org>
Tested-by: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
1 file changed