migration: Fix incorrect integer->float conversion caught by clang

Clang does not like qmp_migrate_set_downtime()'s code to clamp double
@value to 0..INT64_MAX:

    qemu/migration/migration.c:2038:24: error: implicit conversion from 'long' to 'double' changes value from 9223372036854775807 to 9223372036854775808 [-Werror,-Wimplicit-int-float-conversion]

The warning will be enabled by default in clang 10. It is not
available for clang <= 9.

The clamp is actually useless; @value is checked to be within
0..MAX_MIGRATE_DOWNTIME_SECONDS immediately before.  Delete it.

While there, make the conversion from double to int64_t explicit.

Signed-off-by: Fangrui Song <i@maskray.me>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[Patch split, commit message improved]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>