tools/rv: Fix substring match when listing container monitors

tools/rv: Fix substring match when listing container monitors

When listing monitors within a specific container (rv list <container>),
the tool incorrectly matched monitors if the requested container name
was only a prefix of the actual container (e.g., 'rv list sche' would
incorrectly list monitors from 'sched:').

Fix this by ensuring the container name is an exact match and is
immediately followed by the ':' separator.

Fixes: eba321a16fc6 ("tools/rv: Add support for nested monitors")
Reviewed-by: Nam Cao <namcao@linutronix.de>
Link: https://lore.kernel.org/r/20260514152055.229162-3-gmonaco@redhat.com
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>