common/admin_socket: simplify command routing

common/admin_socket: simplify command routing

Back in e30e937c8962249af283a7571eb106ef444b79e3 we made it possible to
route a command via any prefix. This worked when we wanted to pass
arguments but were just dealing with a vector<string>.  These days we have
an actual prefix followed by named arguments, so we don't need this
ad hoc routing.

Derive the prefix from the cmddesc at registration time, and match that
explicitly against the prefix at execution time.

Signed-off-by: Sage Weil <sage@redhat.com>