OvmfPkg NvVarsFileLib: Set NvVars variable after writing vars file

The volatile 'NvVars' variable indicates that the variables do
not need to be loaded from the file again. After we write the
variables out to the file, there is clearly no need to load
them back from the file.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Michael Chang <mchang@suse.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/OvmfPkg@14613 6f19259b-4bc3-4df7-8a09-765794883524