Make "snabbvmx top" print out only counters that are defined

[dpino]

Issue #975 reported a bug in command snabbvmx top.

One bug was related with a typo. Another was related with opening a non existing counter.

The issue was that snabbvmx top attempted to open in-ipv4-frag-reassembled but that counter didn't exist on /var/run/snabb/PID/apps/lwaftr/counters. I modified snabbvmx topto handle the case when tries to open a counter that is not defined.

I think the former code was working before because all counters were created regardless they were being used or not, but now it's not the case.

Fixes #975.

[dpino]

Running instance:

lwaftr (rx/tx/txdrop in Mpps)                       rx      tx      rxGb    txGb    txdrop
 lwaftr_v6                                           0.16    0.16    0.72    0.78    0.162993
 lwaftr_v4                                           0.16    0.16    0.72    0.75    0.162993

                                                                   Total           per second
 lwaftr_v6 in-ipv6-packets                                     1,679,096              163,200
 lwaftr_v6 in-ipv6-bytes                                     923,502,800           89,760,000
 lwaftr_v6 out-ipv6-packets                                    1,679,096              163,200
 lwaftr_v6 out-ipv6-bytes                                  1,004,099,408           97,593,600
 lwaftr_v6 drop-all-ipv6-iface-packets                         1,679,096              163,200

                                                                   Total           per second
 nic ifInDiscards                                                      0                    0

                                                                   Total           per second
 lwaftr_v4 in-ipv4-packets                                     1,760,901              163,200
 lwaftr_v4 in-ipv4-bytes                                     968,495,550           89,760,000
 lwaftr_v4 out-ipv4-packets                                    1,760,901              163,200
 lwaftr_v4 out-ipv4-bytes                                  1,017,800,778           94,329,600
 lwaftr_v4 drop-all-ipv4-iface-packets                         1,760,901              163,200