qemu: add PACKAGECONFIG for capstone

Autobuilder workers were non-deterministically enabling capstone
depending on whether the worker had libcapstone installed.

Add PACKAGECONFIG for capstone with default off, since qemu does not
require capstone support.

Qemu version in dunfell has capstone in the source tree as a submodule
and has configure options to enable it using that source code or using
the system libcapstone.

Qemu versions in master and kirkstone have removed the capstone
submodule and configure options, but added libcapstone autodetection to
meson.

In all cases using PACKAGECONFIG will allow a deterministic build.

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 052ef1f1)
Signed-off-by: Steve Sakoman <steve@sakoman.com>