tests: balloon: Find sshd processes before adjusting OOM scores

The guest images do not create /run/sshd.pid, so the existing helper
invokes choom without a PID and leaves sshd exposed to OOM kills.

Discover all sshd processes with pidof and update each OOM score to
-1000, excluding it from OOM-killer selection.

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>