ArmVirtPkg/ArmVirtQemu: enable the IPv6 stack

Add the IPv6 stack to ArmVirtQemu with a cumulative port of the following
OvmfPkg commits:

* 36c6413f76e5 "OvmfPkg: enable the IPv6 support", 2014-12-19

* 96302b80d90e "OvmfPkg: Enable Network2 Shell Commands for IPv6",
               2016-03-08

* 6d0f8941bdc2 "OvmfPkg: always resolve OpenSslLib, IntrinsicLib and
               BaseCryptLib", 2017-01-17

* 32e22f20c985 "OvmfPkg: correct the IScsiDxe module included for the IPv6
               stack", 2017-01-17

The IPv6-enabled IScsiDxe driver depends on BaseCryptLib, and the
"CryptoPkg/Library/BaseCryptLib/BaseCryptLib.inf" instance depends on
IntrinsicLib and OpensslLib. This is why commit 6d0f8941bdc2 is relevant.

However, unlike in OvmfPkg, in ArmVirtPkg we'll precisely track the
firmware features that require these library classes. (The OvmfPkg
discussion was quite complex, and the OvmfPkg solution was a compromise:
<http://mid.mail-archive.com/1484569332-13440-1-git-send-email-jiaxin.wu@intel.com>.)

The ArmVirtXen platform is not extended with the relevant drivers because
currently it doesn't include any networking support.

Cc: Julien Grall <julien.grall@linaro.org>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1007
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>