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

lib/libvmmapi/vmmapi.c

@@ -133,7 +133,7 @@ vm_open(const char *name)
 
 	return (vm);
 err:
-	vm_destroy(vm);
+	free(vm);
 	return (NULL);
 }
 

usr.sbin/bhyvectl/bhyvectl.c

@@ -1950,7 +1950,9 @@ main(int argc, char *argv[])
 	if (!error) {
 		ctx = vm_open(vmname);
 		if (ctx == NULL) {
-			printf("VM:%s is not created.\n", vmname);
+			fprintf(stderr,
+			    "vm_open: %s could not be opened: %s\n",
+			    vmname, strerror(errno));
 			exit (1);
 		}
 	}