bhyvectl: print a better error message when vm_open() fails

libvmm: explicitly save and restore errno in vm_open()

Use errno to print a more descriptive error message when vm_open() fails

PR: 250671
Reviewed by: grehan
Differential Revision: https://reviews.freebsd.org/D29109

(cherry picked from commit 6bb140e3ca895a148f32c93d50f93619bf735f73)
(cherry picked from commit a7f81b488df2d4a5dcd785b4112e04ffb6ca0442)