network: Trim the bridge name to 15 characters
network: Trim the bridge name to 15 characters

Network interface names in Linux are limited to 15 characters. This
change ensures that the limit is not exceeded.

Fixes: #749
Signed-off-by: Michal Rostecki <82ebf970c69340aebb770c3853be5fd96a3eb584@opensuse.org>

dmacvicar/terraform-provider-libvirt,dmacvicar/terraform-provider-libvirt,dmacvicar/terraform-provider-libvirt