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.

PR: 250671
MFC after: 3 days
Submitted by: marko@apache.org
Reviewed by: grehan
Differential Revision: https://reviews.freebsd.org/D29109