migration: Remove postcopy implications in should_send_vmdesc()

migration: Remove postcopy implications in should_send_vmdesc()

should_send_vmdesc() has a hack inside (which was not reflected in the
function name) in that it tries to detect global postcopy state and that
will affect the value to be returned.

It's easier to keep the helper simple by only check the suppress-vmdesc
property.  Then:

  - On the sender side of its usage, there's already in_postcopy variable
    that we can use: postcopy doesn't send vmdesc at all, so directly skip
    everything for postcopy.

  - On the recv side, when reaching vmdesc processing it must be precopy
    code already, hence that hack check never used to work anyway.

No functional change intended, except a trivial side effect that QEMU
source will start to avoid running some JSON helper in postcopy path, but
that would only reduce the postcopy blackout window a bit, rather than any
other bad side effect.

Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20250114230746.3268797-2-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>