BaseTools GenFv: Report the correct spare FV image size

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2790



If the top FFS is placed in FV image, current FV will show there is no space.
In fact, the pad ffs in FV image can be regarded as the spare space.
This change reports the max pad ffs size as the spare space for use.

Signed-off-by: Liming Gao <liming.gao@intel.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>