Fix timeouts in backup tests

[aesteve-rh]

These tests run a real vm, and github does not support hardware acceleration, so we run with emulation (tcg). This is very slow, so the host running the test is overloaded, it can time out.

Fails in test_incremental_backup_guest take ~20 seconds, using 60 seconds timeout. Using 120 seconds will likely eliminate these random failures

If not, maybe this is a bug in the code communicating with the vms using serial port: https://github.com/oVirt/ovirt-imageio/blob/e189066a0cee0fe8907a01febe26a6ec87ef8692/test/qemu.py#L100

Ideally, we need to log what we see on the wire in debug level, so when this code fails, we can understand if the issue was unexpected input from the guest or too slow guest.