bhyve: Use a blocking socket in the helper process

The send_packet callback does not handle EAGAIN, and on the recv side we
already use poll() and MSG_DONTWAIT to implement a non-blocking loop.

PR: 291616
Tested by: novel
Fixes: 0e62ebd20172 ("bhyve: Move the slirp backend out into a separate process")
Differential Revision: https://reviews.freebsd.org/D54340