net: properly handle illegal fd/vhostfd from command line

When hanlding fd/vhostfd form command line through net_handle_fd_param(),
we need to check mon and return value of strtol() otherwise we could
get segmentation fault or invalid fd when user type an illegal fd/vhostfd.

This patch is based on the suggestions from
Luiz Capitulino <lcapitulino@redhat.com>.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>