ram.c: Do not call save_page_header() from compress threads

ram.c: Do not call save_page_header() from compress threads

save_page_header() accesses several global variables, so calling it
from multiple threads is pretty ugly.

Instead, call save_page_header() before writing out the compressed
data from the compress buffer to the migration stream.

This also makes the core compress code more independend from ram.c.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>