migration: Support periodic RAMBlock dirty bitmap sync

When VM is configured with huge memory, the current throttle logic
doesn't look like to scale, because migration_trigger_throttle()
is only cal

migration: Support periodic RAMBlock dirty bitmap sync

When VM is configured with huge memory, the current throttle logic
doesn't look like to scale, because migration_trigger_throttle()
is only called for each iteration, so it won't be invoked for a long
time if one iteration can take a long time.

The periodic dirty sync aims to fix the above issue by synchronizing
the ramblock from remote dirty bitmap and, when necessary, triggering
the CPU throttle multiple times during a long iteration.

This is a trade-off between synchronization overhead and CPU throttle
impact.

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/f61f1b3653f2acf026901103e1c73d157d38b08f.1729146786.git.yong.huang@smartx.com
[peterx: make prev_cnt global, and reset for each migration]
Signed-off-by: Peter Xu <peterx@redhat.com>

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migration: Move cpu-throttle.c from system to migration

Move cpu-throttle.c from system to migration since it's
only used for migration; this makes us avoid exporting the
util functions and variable

migration: Move cpu-throttle.c from system to migration

Move cpu-throttle.c from system to migration since it's
only used for migration; this makes us avoid exporting the
util functions and variables in misc.h but export them in
migration.h when implementing the periodic ramblock dirty
sync feature in the upcoming commits.

Since CPU throttle timers are only used in migration, move
their registry to migration_object_init.

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/c1b3efaa0cb49e03d422e9da97bdb65cc3d234d1.1729146786.git.yong.huang@smartx.com
[peterx: Fix build on MacOS on cocoa.m, not move cpu-throttle.h yet]
[peterx: Fix subject spelling, per pm215]
Signed-off-by: Peter Xu <peterx@redhat.com>

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