backup: Add support for passing server socket file descriptor to backup NBD server

backup: Add support for passing server socket file descriptor to backup NBD server

In deployments where libvirt is containerized together with the VM it
may be hard for the management application to access listening sockets
inside the container from the outside.

This patch implements "transport='fd'" for the NBD server definition for
backups which allows to use the existing "virDomainFDAssociate()" to
pass FD to a pre-opened server socket to qemu instead of trying to
create it by qemu.

Add schema, enable the parser, add formatter and implement the actual
passing for the qemu backup code.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Spellchecked-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>