OvmfPkg: add missing braces to aggregate and/or union initializers

Lack of these braces causes build errors when -Wno-missing-braces is
absent. Spelling out more braces also helps understanding the code.

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: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/OvmfPkg@15586 6f19259b-4bc3-4df7-8a09-765794883524