aen_cond is used with pthread_cond_wait() and pthread_cond_signal() but
never initialized. That works where a zeroed pthread_cond_t is
initialized on first use; elsewhere pthread_cond_wait() fails at once
and aen_thr() spins with aen_mtx held.

Signed-off-by: Wanpeng Qian <wanpengqian@gmail.com>
Sponsored by: keelos.dev

Built usr.sbin/bhyve on main 7cb4d4dfd with and without the change, plain
and WITH_BHYVE_SNAPSHOT, -Werror: 0 warnings.

FreeBSD's libthr initializes a NULL (PTHREAD_COND_INITIALIZER)
condition variable on first use, so the missing pthread_cond_init() has
no visible effect here; on systems where pthread_cond_t is a structure
the first wait or signal uses garbage.

FreeBSD main guest with -s 10,nvme,<1 GB file>, both versions: nvme0
attaches, "nvmecontrol logpage -p 1" reports no errors, 64 MB written to
nda0 and read back with the same MD5, no errors in dmesg.