Process pending work while waiting for initial kick-off in TCG mode

When the TCG thread is started but not yet the machine, we wait in
qemu_tcg_cpu_thread_fn on tcg_halt_cond. To allow run_on_cpu already at
this time, we need to process pending request in that loop.

CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
