bhyve: Avoid shadowing global variables in bhyverun.c

• Rename the global cores/sockets/threads to cpu_cores/sockets/threads. This way, num_vcpus_allowed() doesn't shadow them.
• The global maxcpus is unused, remove it for the same reason.

(cherry picked from commit 3b6cb9b43657d8898c165c5cd26459e4646ba4f0)