OvmfPkg: Update DSC/FDF to use NetworkPkg's include fragment file.

OvmfPkg: Update DSC/FDF to use NetworkPkg's include fragment file.

This patch updates the platform DSC/FDF files to use the include fragment
files provided by NetworkPkg.
The feature enabling flags in [Defines] section have been updated to use
the NetworkPkg's terms, and the value has been overridden with the original
default value on this platform.

v2:1.Make the comments before Network definition align other parts.
   2.Set NETWORK_ALLOW_HTTP_CONNECTIONS true.
   3.Remove TcpIoLib in lib classes section.
   4.Withdraw the removal of [PcdsFixedAtBuild.X64].

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Message-Id: <20190516081810.27840-2-shenglei.zhang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1293
[lersek@redhat.com: add TianoCore Bugzilla reference]