There is a known bug with QEMU 2.7 which is affecting installations of LAVA using QEMU from jessie-backports.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=840853
The problem manifests in the apply-overlay-guest action because QEMU is unable to process the GuestFS operations due to a missing file which is in the seabios 1.9 package but not in seabios 1.7 - so V2 QEMU test jobs hang at this point.
If you are using QEMU from jessie (version 1:2.1+dfsg-12+deb8u6) then there is no problem and nothing needs to be done.
If you are using QEMU from jessie-backports (version 1:2.7+dfsg-3~bpo8+2 currently) then there are two simple remedies.
0: You can find the updated seabios package in the LAVA production repository. See https://validation.linaro.org/static/docs/v2/installing_on_debian.html#lava-... If you have the production repository already available, you can simply apt update and apt upgrade to install the new seabios package.
1: If you are not using aarch64 QEMU tests or the QEMU UEFI support for LuvOS, then the version of qemu in jessie will work just fine. To downgrade, specify the version string to apt:
$ sudo apt install qemu-system-x86=1:2.1+dfsg-12+deb8u6 qemu-system-common=1:2.1+dfsg-12+deb8u6
It is also worth downgrading other qemu packages at the same time. Check your current install with:
$ dpkg -l | grep qemu
The relevant bug has been re-opened and a backport of seabios has been uploaded. An updated backport of qemu has also been requested which would require the installation of the backported seabios 1.9. So if this bug is not troubling you immediately, you can simply wait for seabios 1.9 to be available in jessie-backports and install it.