OVMF: Add support for more persistent NV variables which can survive a system reboot.

OVMF: Add support for more persistent NV variables which can survive a system reboot.

Make use of EMU Variable driver's PcdEmuVariableNvStoreReserved to allow
NV variables to persist a VM system reset.  The contents of the NV variables
will still be lost when the VM is shut down, but they appear to persist
when the efi shell reset command is run.  (Tested with QEMU 0.10.0.)

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@9241 6f19259b-4bc3-4df7-8a09-765794883524