IOSXR: fix "show route ipv4" parser for hops without interface

[vincentbernat]

"show route ipv4" may not provide an interface for each hop, like in
this example:

B    10.0.0.0/8 [20/30] via 172.19.165.189, 3w0d
                [20/0] via 172.30.254.254, 3w0d

Fix the regex to handle this case correctly. Previously, 3w0d was
considered to be the interface. The regex was stolen from the IPv6
case which was correct (and tested).

Checklist:

• I have updated the changelog.
• I have updated the documentation (If applicable).
• I have added tests to cover my changes (If applicable).
• All new and existing tests passed.
• All new code passed compilation.

[tahigash]

thanks for the fix. left one comment.

[d-the-iii]

Fix looks good to me, but I agree with Takashi. Do you have an example of this line without the interface so we can manually test the regex, and to make unit tests with?

[tahigash]

thanks for addressing comments and contribution!