memory: Unify the definiton of ReplayRamPopulate() and ReplayRamDiscard()

memory: Unify the definiton of ReplayRamPopulate() and ReplayRamDiscard()

Update ReplayRamDiscard() function to return the result and unify the
ReplayRamPopulate() and ReplayRamDiscard() to ReplayRamDiscardState() at
the same time due to their identical definitions. This unification
simplifies related structures, such as VirtIOMEMReplayData, which makes
it cleaner.

Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Link: https://lore.kernel.org/r/20250612082747.51539-4-chenyi.qiang@intel.com
Signed-off-by: Peter Xu <peterx@redhat.com>