ui/clipboard: clear deferred serial reset flag on resume

cb_reset_serial_on_resume defers a clipboard serial reset until the
machine runs again. qemu_clipboard_change_state() performs that reset
on resume, but leaves the flag set.

As a result, every later transition back to RUNNING triggers another
QEMU_CLIPBOARD_RESET_SERIAL notification even when no reset is pending.
That causes unnecessary reset handling in clipboard backends such as
vdagent and dbus.

Clear the deferred flag before resetting so the reset is performed only
once for each deferred request.

Fixes: c967ff606b99 ("ui/clipboard: delay clipboard update when not running")
Signed-off-by: GuoHan Zhao <zhaoguohan@kylinos.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20260416070403.1683648-1-zhaoguohan@kylinos.cn>