OvmfPkg: PlatformDxe: introduce state for the main form

OvmfPkg: PlatformDxe: introduce state for the main form

We'll need a C language (ie. structure) representation for the state of
the visual elements on the form. We choose the Buffer Storage kind (see
29.2.5.6 "Storage" in UEFI 2.4A), because it's easy to work with.

Note that the structure added in this patch has nothing to do with UEFI
non-volatile variables.

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@15368 6f19259b-4bc3-4df7-8a09-765794883524