Fix coverity CID: 1391235, 1193654 and 1193651 (Unchecked return value)

[araujo]

Summary

pthread_rwlock_unlock(3) returns 0 if successful, otherwise an error number
will be returned to indicate the error, so I'm applying an assert(3) to do
a sanity check of the return value.

Test Plan

Rebuild bhyve with this patch:

1. Test FreeBSD HEAD as a guest.
2. Test FreeBSD HEAD as a guest with "wire guest memory -S".