Fix errno check, prevent spurious errors under heavy load

Fix errno check, prevent spurious errors under heavy load

From man poll(2), poll does not set errno=EAGAIN on interrupt, however
it does set errno=EINTR. Have libvirt retry on the appropriate errno.

Under heavy load, a program of mine kept getting libvirt errors 'poll on
socket failed: Interrupted system call'. The signals were SIGCHLD from
processes forked by threads unrelated to those using libvirt.