wic: make sure that partition size is always an integer in internal processing

wic: make sure that partition size is always an integer in internal processing

The size field of Partition class is expected to be an integer and ought
to be set inside prepare_*() method. Make sure that this is always the
case.

(From OE-Core rev: a37838f995ae642b0b8bdd47a605a264fb3bf200)

Signed-off-by: Maciej Borzecki <maciej.borzecki@rndity.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>