conf: Add 'fdgroup' attribute for 'file' disks

conf: Add 'fdgroup' attribute for 'file' disks

The 'fdgroup' will allow users to specify a passed FD (via the
'virDomainFDAssociate()' API) to be used instead of opening a path.
This is useful in cases when e.g. the file is not accessible from inside
a container.

Since this uses the same disk type as when we open files via names this
patch also introduces a hypervisor feature which the hypervisor asserts
that code paths are ready for this possibility.

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