tests/vm: Allow to set qemu-img path

tests/vm: Allow to set qemu-img path

By default VM build test use qemu-img from system's PATH to
create the image disk. Due the lack of qemu-img on the system
or the desire to simply use a version built with QEMU, it would
be nice to allow one to set its path. So this patch makes that
possible by reading the path to qemu-img from QEMU_IMG if set,
otherwise it fallback to default behavior.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20191114134246.12073-2-wainersm@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>