test/py: Fix a problem with setup_image

While we can be passed an image size to use, we always called qemu-img
with 20M as the size. Fix this by using the size parameter.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/test/py/tests/fs_helper.py b/test/py/tests/fs_helper.py
index f3e81b6..2faab21 100644
--- a/test/py/tests/fs_helper.py
+++ b/test/py/tests/fs_helper.py
@@ -95,7 +95,7 @@
 
     try:
         check_call(f'mkdir -p {mnt}', shell=True)
-        check_call(f'qemu-img create {fname} 20M', shell=True)
+        check_call(f'qemu-img create {fname} {img_size}M', shell=True)
         check_call(f'printf "{spec}" | sfdisk {fname}', shell=True)
     except CalledProcessError:
         call(f'rm -f {fname}', shell=True)