memattrs: Convert unspecified member to bool

memattrs: Convert unspecified member to bool

Convert `unspecified` member of MemTxAttrs from bit field to bool, so
that bindgen could generate more ergonomic Rust binding with bool type.

As a result, MemTxAttrs needs to be expanded from 4 bytes to 8 bytes.

Therefore, move `unspecified` to after the bit fields and add reserved
members to ensure that the whole structure is packed into 8 bytes.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250121151322.171832-2-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>