fw_cfg: Splash image loader can overrun a stack variable, fix

read_splashfile() passes the address of an int variable as size_t *
parameter to g_file_get_contents(), with a cast to gag the compiler.

No problem on machines where sizeof(size_t) == sizeof(int).

Happens to work on my x86_64 box (64 bit little endian): the least
significant 32 bits of the file size end up in the right place
(caller's variable file_size), and the most significant 32 bits
clobber a place that gets assigned to before its next use (caller's
variable file_type).

I'd expect it to break on a 64 bit big-endian box.

Fix up the variable types and drop the problematic cast.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
