NetworkPkg/Ip6Dxe: Fix the IPv6 PXE boot option goes missing issue

NetworkPkg/Ip6Dxe: Fix the IPv6 PXE boot option goes missing issue

This patch is to fix the potential issue recorded at Bugzilla 636:
https://bugzilla.tianocore.org/show_bug.cgi?id=636

The issue is caused by the IPv6 policy switching after PXEv6 boot. When IP
policy is changing, the IPv6 children used by PXE.UdpRead() will be destroyed.
Then, PXE Stop() function is called to uninstall the devicePath protocol,
which leads to the IPv6 PXE boot option goes missing.

Through the above analysis, the IP driver should take the responsibility for
the upper layer network stacks recovery by using ConnectController().

Cc: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>
Cc: Subramanian Sriram <sriram-s@hpe.com>
Cc: Ni Ruiyu <ruiyu.ni@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>
Reviewed-by: Subramanian Sriram <sriram-s@hpe.com>
Tested-by: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>