qemu: Propagate shared_filesystems

qemu: Propagate shared_filesystems

virFileIsSharedFS() is the function that ultimately decides
whether a filesystem should be considered shared, but the list
of manually configured shared filesystems is part of the QEMU
driver's configuration, so we need to pass the information
through several layers in order to make use of it.

Note that with this change the list is propagated all the way
through, but its contents are still ignored, so the behavior
remains the same for now.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>