OvmfPkg/PlatformBootManagerLib: Postpone the shell registration

We currently register the shell before creating the boot options for
the block devices and the network devices, so the boot manager boots
into the internal shell if the user doesn't specify the boot order.
However, Xen doesn't support fw_cfg, so there is no way to change the
boot order with the external command, and the firmware will always
boot into the internal shell if the user doesn't interfere the boot
process.

This patch postpones the shell registration after MdeModulePkg/BDS
creates all the boot options for the block and network devices, so
that firmware will try to boot the block/network devices first.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>