virttest.qemu_vm: Replace qemu_dst_binary on migrate

[ldoktor]

For years we had been claiming ping-pong migration, but in reality we do ping->pong->pong->pong migrations as the source qemu_binary is only used once. Let's replace the qemu_dst_binary for qemu_binary of the source VM in order to start switching between ping and pong binaries.

Signed-off-by: Lukáš Doktor [email protected]

[luckyh]

I certainly know that we support migrating VMs from older QEMUs to newer ones, does it work for the reverse side?

[ldoktor]

Officially we don't support it, but we do want to test it to uncover bugs. Also note that src and dst doesn't necessarily mean older and newer qemu. In my migration matrices I use various combinations.

[ldoktor]

@luckyh I talked to Dave and as a matter of fact we do support migrating there and back as well. So this actually adds coverage that is currently missing in Avocado-vt.