libvmmapi: Fix warnings and stop overridding WARNS

• Avoid shadowing the global optarg.
• Sprinkle __unused.
• Cast nitems() to int.
• Fix sign in vm_copy_setup().

Reviewed by: grehan
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31306