util: allow sending mac addr to virNetNewLink without ifindex

util: allow sending mac addr to virNetNewLink without ifindex

Although until now, any use of the extra_args argument (a pointer to a
struct containing extra attributes to add the the RTM_NEWLINK message)
would always have the ifindex and mac set, so the code could assume it
was safe to add both to the message if extra_args != NULL. There is
now a use for setting a MAC address in the RTM_NEWLINK without setting
the ifindex, so we should check each of these separately.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>