vircommand: Ensure buffers are NULL-terminated

vircommand: Ensure buffers are NULL-terminated

The memory allocated by VIR_REALLOC_N() is uninitialized,
which means it's not possible to figure out whether any
output was produced at all after the fact.

Since we don't care about the previous contents of buffers,
if any, use VIR_FREE() followed by VIR_ALLOC_N() instead.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>