OvmfPkg: raise DXEFV size to 8 MB

This fixes build errors like:

  GenFds.py...
    the required fv image size 0x71b118 exceeds the set fv image size
    0x700000

which is reported at least for:
(a) -b DEBUG -D SECURE_BOOT_ENABLE -t GCC44,
(b) -b DEBUG -D SECURE_BOOT_ENABLE -t GCC48 -D CSM_ENABLE

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@15309 6f19259b-4bc3-4df7-8a09-765794883524