Use ADD and git-url fragment to clone qemu repo, use master as defaul… #658

Job	Run time
prepare (mainline)	7m 2s
prepare (buildkit)	7m 1s
prepare (desktop)	7m 1s
validate (./hack/lint)	27s
test (mainline)	3m 47s
e2e (mainline, pip-youtube-dl, linux/arm/v7)	1m 17s
validate (./hack/validate-vendor)	17s
test (buildkit)	28s
validate (./hack/install-and-test)	51s
e2e (mainline, webpack, linux/arm/v7)	1m 56s
test (desktop)	6s
e2e (desktop, pip-youtube-dl, linux/arm/v7)	1m 22s
e2e (desktop, webpack, linux/arm/v7)	1m 43s
e2e (mainline, ejabberd, linux/arm64)	13m 33s
e2e (desktop, tini, linux/arm64, --rm)	36s
e2e (desktop, postgis, linux/arm64, -d --name postgis, postgis, UTC [1] LOG: database system is ...	40s
build (mainline)	32m 3s
build (buildkit)	30m 36s
build (desktop)	32m 12s
	2h 22m 58s

Provide feedback