mac_prefix: do not allow multicast prefixes

mac_prefix: do not allow multicast prefixes

MAC-addresses having the LSB of the first octet set, are considered
multicast-addresses (see [0,1]). LXC (the kernel) does not allow
such a mac-address to be set for a device, thus preventing containers from
starting if a multicast prefix is set (reported in [2] by Alexandre)

This patch introduces 'mac-prefix' (permitting only unicast prefixes) via
register_format and uses it instead of the pattern.

[0] https://lists.linuxcontainers.org/pipermail/lxc-users/2010-August/000783.html
[1] https://en.wikipedia.org/wiki/MAC_address
[2] https://pve.proxmox.com/pipermail/pve-devel/2019-March/035996.html

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>