virsh: improve option handling

virsh: improve option handling

The documentation for vshCommandOptString claims that it returns
-1 on a missing required argument, but in reality, that error
message was unreachable (it was buried inside an if clause that
is true only if the argument was present).  The code was so hairy
that I decided a rewrite would make it easier to understand,
and actually return the error values we want.

Meanwhile, our construction guarantees that all vshCmdOpt have
a non-null def member, so there are some redundant checks that
can be trimmed.

* tools/virsh.c (vshCommandOpt): Alter signature.
(vshCommandOptInt, vshCommandOptUInt, vshCommandOptUL)
(vshCommandOptString, vshCommandOptLongLong)
(vshCommandOptULongLong, vshCommandOptBool): Adjust all callers.
(vshCommandOptArgv): Remove dead condition.