kvm: Fix race between timer signals and vcpu entry under !IOTHREAD

Found by Stefan Hajnoczi: There is a race in kvm_cpu_exec between
checking for exit_request on vcpu entry and timer signals arriving
before KVM starts to catch them. Plug it by blocking both timer related
signals also on !CONFIG_IOTHREAD and process those via signalfd.

As this fix depends on real signalfd support (otherwise the timer
signals only kick the compat helper thread, and the main thread hangs),
we need to detect the invalid constellation and abort configure.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
CC: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
