config: NIC macaddr: enforce unicast MAC addresses

config: NIC macaddr: enforce unicast MAC addresses

creating a VM with a NIC with multicast mac (see [1]) is possible, but setting
the interface's link up inside the guest fails (tested on Debian stable).
The issue was noted with LXC first (see [0,2]) and then tested with Qemu.

This patch uses the 'mac-addr' standard_option defined in PVE::JSONSchema to
ensure only unicast MAC addresses are used for netconfig.

[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>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>