vmstate: Update max_elems early and check field compressable once

QEMU has a trick in vmstate_save_vmsd_v(), where it will try to compress multiple JSON entries into one with a count to avoid duplicated entries. That only applies to the cases where vmsd_can_compress() should return true. For example, vmsd_desc_field_start() later (who will take the updated max_elems as the last parameter) will ignore the value passed in when vmsd_can_compress() returns false. Do that check once at the start of loop, and use it to update max_elems, so that max_elems keeps 1 for uncompressable VMSD fields, which is more straightforward. This also paves way to make this counter work for ptr marker VMSD fields too. No functional change intended in this patch alone. Reviewed-by: Fabiano Rosas <farosas@suse.de> Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io> Signed-off-by: Peter Xu <peterx@redhat.com> Reviewed-by: Juraj Marcin <jmarcin@redhat.com> Link: https://lore.kernel.org/qemu-devel/20260401202844.673494-5-peterx@redhat.com Signed-off-by: Fabiano Rosas <farosas@suse.de>