qemu: Prepare data for FD-passed disk image sources

qemu: Prepare data for FD-passed disk image sources

When starting up a VM with FD-passed images we need to look up the
corresponding named FD set and associate it with the virStorageSource
based on the name.

The association is brought into virStorageSource as security labelling
code will need to access the FD to perform selinux labelling.

Similarly when startup is complete in certain cases we no longer need to
keep the copy of FDs and thus can close them.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>