libvirt-qemu: Don't allow NULL cmd in virDomainQemuMonitorCommandWithFiles()

libvirt-qemu: Don't allow NULL cmd in virDomainQemuMonitorCommandWithFiles()

Nothing in daemon code is prepared for the command in
virDomainQemuMonitorCommandWithFiles() to be NULL. In fact, the
client side doesn't expect this either as our RPC describes the
argument as:

    remote_nonnull_string cmd;

Validate the argument in the public API implementation.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>