OvmfPkg/PlatformBootManagerLib: keep the logo after connecting devices

OVMF (unlike ArmVirtPkg) has traditionally cleared the screen after
connecting devices. This is not really necessary, and keeping the logo up
while the progress bar is advancing at the bottom looks great. So don't
clear the screen.

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>