During a XenServer reboot or hotfix installation when there are multiple XenServers in the pool, you may run into an issue where the NICs seem to disappear. Also, if you try to run XE commands, you may get the error "The host is still booting". In the cases I have seen this issue, the problem is what the problematic XenServer thinks is the pool master when in fact the pool master is a different host. Below are the steps I went through to diagnose and resolve this issue:
Go to the host's command-line interface (CLI).
Type ifconfig and verify the underlying system sees the NICs and that they are up.
Navigate to \var\log\xensource.log and review the log entries. In my case, it was stating that it was trying to contact the pool master but was not receiving a response. I knew my pool master was up and functional as the other XenServer hosts did not have an issue.
Navigate to \xensource\pool.conf and modify the IP address found inside the file to the current pool master IP.
At the CLI, type xe-toolstack-restart.