spapr: fix memory hot-unplugging

If, once the kernel has booted, we try to remove a memory
hotplugged while the kernel was not started, QEMU crashes on
an assert:

    qemu-system-ppc64: hw/virtio/vhost.c:651:
                       vhost_commit: Assertion `r >= 0' failed.
    ...
    #4  in vhost_commit
    #5  in memory_region_transaction_commit
    #6  in pc_dimm_memory_unplug
    #7  in spapr_memory_unplug
    #8  spapr_machine_device_unplug
    #9  in hotplug_handler_unplug
    #10 in spapr_lmb_release
    #11 in detach
    #12 in set_allocation_state
    #13 in rtas_set_indicator
    ...

If we take a closer look to the guest kernel log, we can see when
we try to unplug the memory:

    pseries-hotplug-mem: Attempting to hot-add 4 LMB(s)

What happens:

    1- The kernel has ignored the memory hotplug event because
       it was not started when it was generated.

    2- When we hot-unplug the memory,
       QEMU starts to remove the memory,
            generates an hot-unplug event,
        and signals the kernel of the incoming new event

    3- as the kernel is started, on the QEMU signal, it reads
       the event list, decodes the hotplug event and tries to
       finish the hotplugging.

    4- QEMU receive the the hotplug notification while it
       is trying to hot-unplug the memory. This moves the memory
       DRC to an invalid state

This patch prevents this by not allowing to set the allocation
state to USABLE while the DRC is awaiting release.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1432382

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
