qemu: Filter cur_balloon ABI check for certain transactions

qemu: Filter cur_balloon ABI check for certain transactions

Since the domain lock is not held during preparation of an external XML
config, it is possible that the value can change resulting in unexpected
failures during ABI consistency checking for some save and migrate
operations.

This patch adds a new flag to skip the checking of the cur_balloon value
and then sets the destination value to the source value to ensure
subsequent checks without the skip flag will succeed.

This way it is protected from forges and is keeped up to date too.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>