OvmfPkg/PlatformBootManagerLib: bring back the progress bar

OvmfPkg/PlatformBootManagerLib: bring back the progress bar

OVMF's Platform BDS used to have a nice progress bar (with
IntelFrameworkModulePkg BDS). We can restore it by copying the
PlatformBootManagerWaitCallback() function verbatim from

  Nt32Pkg/Library/PlatformBootManagerLib/PlatformBootManager.c

It can be tested by passing the following option to QEMU (5 seconds):

  -boot menu=on,splash-time=5000

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: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>