Storing the motion count in uint16_t and not in uint32_t since
the exact count is not important, just its division in
SPICE_INPUT_MOTION_ACK_BUNCH (see the next 2 patches).
The file will hold the declarations of the different migration
data messages (depending on the channel), that will be passed
from the src server to the dst server, via the client, using
SPICE_MSG_MIGRATE_DATA.
