qemu-xen-trad/block: use a character device if a block device is given

qemu-xen-trad/block: use a character device if a block device is given

On NetBSD a userland process is better with the character device
interface. In addition, a block device can't be opened twice; if a Xen
backend opens it, qemu can't and vice-versa.

This is a backport of 1de1ae0a7d956b3c87712bf2c09d277f99873f4c.

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Backport-requested-by: Roger Pau Monne <roger.pau@citrix.com>
Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>