OvmfPkg: raise DXEFV size to 12 MB

OvmfPkg: raise DXEFV size to 12 MB

Similarly to the "cadence" mentioned in commit d272449d9e1e ("OvmfPkg:
raise DXEFV size to 11 MB", 2018-05-29), it's been ~1.75 years, and we've
outgrown DXEFV again. Increase the DXEFV size to 12MB now.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Gary Lin <glin@suse.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2585
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200310175025.18849-1-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>