OvmfPkg: raise DXEFV size to 10 MB

OvmfPkg: raise DXEFV size to 10 MB

We reached the size limit again.
Building OVMF with the following command

$ ./OvmfPkg/build.sh -D SECURE_BOOT_ENABLE -D NETWORK_IP6_ENABLE -D HTTP_BOOT_ENABLE

and it ended up with

GenFds.py...
GenFv: ERROR 3000: Invalid
 : error 7000: Failed to generate FV
  the required fv image size 0x900450 exceeds the set fv image size 0x900000

Since the new UEFI features, such as HTTPS, are coming, we need a
larger DEXFV eventually.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>