migration/ram.c: do not set 'postcopy_running' in POSTCOPY_INCOMING_END

When migrating a VM with 'migrate_set_capability postcopy-ram on'
a postcopy_state is set during the process, ending up with the
state POSTCOPY_INCOMING_END when the migration is over. This
postcopy_state is taken into account inside ram_load to check
how it will load the memory pages. This same ram_load is called when
in a loadvm command.

Inside ram_load, the logic to see if we're at postcopy_running state
is:

postcopy_running = postcopy_state_get() >= POSTCOPY_INCOMING_LISTENING

postcopy_state_get() returns this enum type:

typedef enum {
    POSTCOPY_INCOMING_NONE = 0,
    POSTCOPY_INCOMING_ADVISE,
    POSTCOPY_INCOMING_DISCARD,
    POSTCOPY_INCOMING_LISTENING,
    POSTCOPY_INCOMING_RUNNING,
    POSTCOPY_INCOMING_END
} PostcopyState;

In the case where ram_load is executed and postcopy_state is
POSTCOPY_INCOMING_END, postcopy_running will be set to 'true' and
ram_load will behave like a postcopy is in progress. This scenario isn't
achievable in a migration but it is reproducible when executing
savevm/loadvm after migrating with 'postcopy-ram on', causing loadvm
to fail with Error -22:

Source:

(qemu) migrate_set_capability postcopy-ram on
(qemu) migrate tcp:127.0.0.1:4444

Dest:

(qemu) migrate_set_capability postcopy-ram on
(qemu)
ubuntu1704-intel login:
Ubuntu 17.04 ubuntu1704-intel ttyS0

ubuntu1704-intel login: (qemu)
(qemu) savevm test1
(qemu) loadvm test1
Unknown combination of migration flags: 0x4 (postcopy mode)
error while loading state for instance 0x0 of device 'ram'
Error -22 while loading VM state
(qemu)

This patch fixes this problem by changing the existing logic for
postcopy_advised and postcopy_running in ram_load, making them
'false' if we're at POSTCOPY_INCOMING_END state.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
CC: Juan Quintela <quintela@redhat.com>
CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reported-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
