qemu: do not attempt to pass unopened vsock FD
qemu: do not attempt to pass unopened vsock FD

On normal vm startup, we open a file descriptor
for the vsock device in qemuProcessPrepareHost.

However, when doing domxml-to-native, no file descriptors are open.

Only pass the fd if it's not -1, to make domxml-to-native work.

https://bugzilla.redhat.com/show_bug.cgi?id=1777212

Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>

jfehlig/libvirt,jfehlig/libvirt,zippy2/libvirt,olafhering/libvirt,libvirt/libvirt,olafhering/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,libvirt/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt