OvmfPkg/BdsPlatform: don't restore NvVars from disk when flash is present

OvmfPkg/BdsPlatform: don't restore NvVars from disk when flash is present

QemuFlashFvbServicesRuntimeDxe provides actual persistent storage for
non-volatile variables. When it is active, any on-disk NvVars file counts
as a stale source of variables -- hence don't load these files in BDS.

This also allows Secure Boot settings (eg. enrolled keys) to survive cold
VM reboots.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14844 6f19259b-4bc3-4df7-8a09-765794883524