migration: do floating-point division

Dividing integer expressions transferred_bytes and time_spent, and then converting
the integer quotient to type double. Any remainder, or fractional part of the
quotient, is ignored.  Fix this.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>