Constant vmm_vhost::message::MAX_ATTACHED_FD_ENTRIES

pub const MAX_ATTACHED_FD_ENTRIES: usize = 32;

The VhostUserMemory message has variable message size and variable number of attached file descriptors. Each user memory region entry in the message payload occupies 32 bytes, so setting maximum number of attached file descriptors based on the maximum message size. But rust only implements Default and AsMut traits for arrays with 0 - 32 entries, so further reduce the maximum number…