migration/multifd: Stop changing the packet on recv side

migration/multifd: Stop changing the packet on recv side

As observed by Philippe, the multifd_ram_unfill_packet() function
currently leaves the MultiFDPacket structure with mixed
endianness. This is harmless, but ultimately not very clean.

Stop touching the received packet and do the necessary work using
stack variables instead.

While here tweak the error strings and fix the space before
semicolons. Also remove the "100 times bigger" comment because it's
just one possible explanation for a size mismatch and it doesn't even
match the code.

CC: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>