tools/hotplug: Fix dhcpd symlink removal in vif-nat

tools/hotplug: Fix dhcpd symlink removal in vif-nat

Copy temp files used to add/remove dhcpd configurations to avoid
replacing potential symlinks.

If dhcp.conf is a symlink pointing to dhcp.conf.real, using 'mv'
creates a new file dhcp.conf where cp will actually modify
dhcp.conf.real instead of replacing the symlink with a real
file.

Using 'cp' prevents some mistakes where the user will actually
continue to modify dhcp.conf.real where it would not be the one
used anymore.

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Wei Liu <wl@xen.org>