util: merge main-loop.c and iohandler.c

util: merge main-loop.c and iohandler.c

main-loop.c has a dependency on iohandler.c, and everything breaks
if that dependency is instead satisfied by stubs/iohandler.c.
Just put everything in the same file to avoid strange dependencies
on the order of files in util-obj-y.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1562952875-53702-1-git-send-email-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>