vma: create: register all streams before entering coroutines

vma: create: register all streams before entering coroutines

Otherwise, the header might already get written by a coroutine and
registering further streams will fail after that.

Also adds a missing g_list_free call for the other GList that's used.

Reported in the community forum:
https://forum.proxmox.com/threads/104744/

Reproducer script (increase beyond 30 if the issue isn't triggered yet):
> #!/usr/bin/perl
>
> my $dir = "./vma-create-bug";
> mkdir $dir;
>
> my $archive_path = "$dir/vzdump-qemu-104-2202_02_02-00_00_00.vma";
> unlink $archive_path;
>
> my $cmd = "vma create $archive_path -v";
> for (my $i = 0; $i < 30; $i++) {
>   system("truncate -s 1M $dir/drive-virtio$i.img");
>   $cmd .= " drive-virtio$i=$dir/drive-virtio$i.img";
> }
> system($cmd);

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>