pc: fix crash in rtc_set_memory() if initial cpu is marked as hotplugged

'hotplugged' propperty is meant to be used on migration side when migrating
source with hotplugged devices.
However though it not exacly correct usage of 'hotplugged' property
it's possible to set generic hotplugged property for CPU using
 -cpu foo,hotplugged=on
or
 -global foo.hotplugged=on

in this case qemu crashes with following backtrace:

...

because pc_cpu_plug() assumes that hotplugged CPU could appear only after
rtc/fw_cfg are initialized.
Fix crash by replacing assumption with explicit checks of rtc/fw_cfg
and updating them only if they were initialized.

Cc: qemu-stable@nongnu.org
Reported-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1483108391-199542-1-git-send-email-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>