bhyvectl: print a better error message when vm_open() fails
Use errno to print a more descriptive error message when vm_open() fails
libvmm: preserve errno when vm_device_open() fails
vm_destroy() squashes errno by making a dive into sysctlbyname() - we
can safely skip vm_destroy() here since it's not doing any critical
clean up at this point. Replace vm_destroy() with a free() call.