OvmfPkg/OvmfXen.dsc: Enable MD5 while enable iSCSI

OvmfPkg/OvmfXen.dsc: Enable MD5 while enable iSCSI

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3003

There is a plan to make MD5 disable as default.
The new MACRO ENABLE_MD5_DEPRECATED_INTERFACES
would be introduced to enable MD5. Make the
definition ahead of the change to avoid build
error after the MACRO changed.

Enable iSCSI.

Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Message-Id: <20201112055558.2348-11-zhichao.gao@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Build-tested-by: Laszlo Ersek <lersek@redhat.com>