docker: Flatten default target list in test-quick

Previously it is expanded to a whitespace separated list which is not
the most appropriate format. Since it's only two items, flatten it.
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <1474429768-25027-6-git-send-email-famz@redhat.com>
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>

tests/docker/test-quick

@@ -13,7 +13,7 @@
 
 . common.rc
 
-DEF_TARGET_LIST="$(echo {x86_64,aarch64}-softmmu)"
+DEF_TARGET_LIST="x86_64-softmmu,aarch64-softmmu"
 TARGET_LIST=${TARGET_LIST:-$DEF_TARGET_LIST} \
 build_qemu
 make check $MAKEFLAGS