vhost-vdpa: allow set feature VHOST_F_LOG_ALL when been negotiated.

virt inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I86ITO



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It's not allowed to change the features after vhost-vdpa devices have
been negotiated. But log start/end is allowed. Add exception to feature
VHOST_F_LOG_ALL.

Signed-off-by: jiangdongxu <jiangdongxu1@huawei.com>