bhyvectl: do not return garbage from send_message

err is used uninitialized in some cases.

Reviewed by: corvink, markj
MFC after: 1 week
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D38886

(cherry picked from commit 89fe7b98fe45cf56d60d0d4dfa1bfad3ba6908ec)