spapr_rng: fix race with main loop

Since commit "60253ed1e6ec rng: add request queue support to rng-random",
the use of a spapr_rng device may hang vCPU threads.

The following path is taken without holding the lock to the main loop mutex:

h_random()
  rng_backend_request_entropy()
    rng_random_request_entropy()
      qemu_set_fd_handler()

The consequence is that entropy_available() may be called before the vCPU
thread could even queue the request: depending on the scheduling, it may
happen that entropy_available() does not call random_recv()->qemu_sem_post().
The vCPU thread will then sleep forever in h_random()->qemu_sem_wait().

This could not happen before 60253ed1e6ec because entropy_available() used
to call random_recv() unconditionally.

This patch ensures the lock is held to avoid the race.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Cédric Le Goater <clg@fr.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
