qga/commands-posix: Support fsinfo for non-PCI virtio devices, too

QEMU on s390x uses virtio via channel I/O instead of PCI by default.
Add a function to detect and provide information for virtio-scsi and
virtio-block devices here, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>