lxd/firewall/drivers: Update ebtables rule matching logic.
lxd/firewall/drivers: Update ebtables rule matching logic.

We are now passing subnets into ebtables in the format <ip>/<mask>
but ebtables prints the rules in CIDR format. This commit generalises
rule matching logic to capture all subnets (and removes the two
hardcoded cases).

Signed-off-by: Mark Laing <e4eb1ed17d42befab1db63da5bbc17c0a2c16113@canonical.com>

stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd