P10 sbefifo backend can not start a stopped thread

[npiggin]

pdbg -b sbefifo -p0 -c12 -t0 stop

pdbg -b sbefifo -p0 -c12 -t0 start

The start command fails and the thread remains quiesced if the thread was inactive (in stop).

The sreset command succeeds in this case. The start command succeeds if the thread was active when stopped (e.g., powersave=off).

When using a scom based backend (https://lists.ozlabs.org/pipermail/pdbg/2020-December/002345.html) everything works correctly.