hide long commandline on vm_start/migrate failure

hide long commandline on vm_start/migrate failure

By default run_command prints the entire commandline executed when an
error occurs, but QEMU and our migrate command are not only
uninteresting to the user[*] but also annoyingly long. Hide them and only
print the exit code.

[*] Especially our migrate command, since it can't be manually executed
anyway. QEMU's commandline *might* contain something interesting, but is
so long that it's tricky to parse anyway, any a user can always call 'qm
showcmd --pretty'.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>