block: avoid SIGUSR2

block: avoid SIGUSR2

Now that iothread is always compiled sending a signal seems only an
additional step. This patch also avoid writing to two pipe (one from signal
and one in qemu_service_io).

Work with kvm enabled or disabled. strace output is more readable (less syscalls).

[ kwolf: Merged build fix by Paolo Bonzini ]

Signed-off-by: Frediano Ziglio <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>