Fix "defined but not used" warning

Fix "defined but not used" warning

The function qemu_calculate_timeout() is only used when CONFIG_IOTHREAD
is not defined. When CONFIG_IOTHREAD is defined, we have the following
warning:

vl.c:4389: warning: ‘qemu_calculate_timeout’ defined but not used

This change fixes that by moving the #ifdef/#endif from main_loop()
into qemu_calculate_timeout(). This encapsulates the logic and allow
us to use qemu_calculate_timeout() when CONFIG_IOTHREAD is defined
or not (suggested by Glauber Costa).

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>