OvmfPkg: raise DXEFV size to 9 MB

OvmfPkg: raise DXEFV size to 9 MB

With gcc5 and enabling SECURE_BOOT and NETWORK_IP6, the build
failed with this error:

GenFv: ERROR 3000: Invalid
  the required fv image size 0x814c18 exceeds the set fv image size 0x800000

Raise the DXEFV size to 9 MB to fix the build error.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Ching-Pang Lin <glin@suse.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18577 6f19259b-4bc3-4df7-8a09-765794883524