migration: Clean up signed vs. unsigned XBZRLE cache-size

migration: Clean up signed vs. unsigned XBZRLE cache-size

73af8dd8d7 "migration: Make xbzrle_cache_size a migration
parameter" (v2.11.0) made the new parameter unsigned (QAPI type
'size', uint64_t in C).  It neglected to update existing code, which
continues to use int64_t.

migrate_xbzrle_cache_size() returns the new parameter.  Adjust its
return type.

QMP query-migrate-cache-size returns migrate_xbzrle_cache_size().
Adjust its return type.

migrate-set-parameters passes the new parameter to
xbzrle_cache_resize().  Adjust its parameter type.

xbzrle_cache_resize() passes it on to cache_init().  Adjust its
parameter type.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Message-Id: <20210202141734.2488076[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>