migration: Make PageSearchStatus part of RAMState

migration: Make PageSearchStatus part of RAMState

We used to allocate PSS structure on the stack for precopy when sending
pages.  Make it static, so as to describe per-channel ram migration status.

Here we declared RAM_CHANNEL_MAX instances, preparing for postcopy to use
it, even though this patch has not yet to start using the 2nd instance.

This should not have any functional change per se, but it already starts to
export PSS information via the RAMState, so that e.g. one PSS channel can
start to reference the other PSS channel.

Always protect PSS access using the same RAMState.bitmap_mutex.  We already
do so, so no code change needed, just some comment update.  Maybe we should
consider renaming bitmap_mutex some day as it's going to be a more commonly
and big mutex we use for ram states, but just leave it for later.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>