migration: Add postcopy-preempt capability

migration: Add postcopy-preempt capability

Firstly, postcopy already preempts precopy due to the fact that we do
unqueue_page() first before looking into dirty bits.

However that's not enough, e.g., when there're host huge page enabled, when
sending a precopy huge page, a postcopy request needs to wait until the whole
huge page that is sending to finish.  That could introduce quite some delay,
the bigger the huge page is the larger delay it'll bring.

This patch adds a new capability to allow postcopy requests to preempt existing
precopy page during sending a huge page, so that postcopy requests can be
serviced even faster.

Meanwhile to send it even faster, bypass the precopy stream by providing a
standalone postcopy socket for sending requested pages.

Since the new behavior will not be compatible with the old behavior, this will
not be the default, it's enabled only when the new capability is set on both
src/dst QEMUs.

This patch only adds the capability itself, the logic will be added in follow
up patches.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220707185342.26794-2-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>