OvmfPkg: Replace obsoleted network drivers from platform DSC/FDF.

OvmfPkg: Replace obsoleted network drivers from platform DSC/FDF.

V2:
Add missed library instance for NetworkPkg iSCSI driver.

This patch replaces the MdeModulePkg TCP, PXE and iSCSI driver with those
ones in NetworkPkg. These 3 drivers in MdeModulePkg are not being actively
maintained and will be removed from edk2 master soon.

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>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>