qemu: Use qemuBuildChrChardevCommand for vhost-user disk backend

qemu: Use qemuBuildChrChardevCommand for vhost-user disk backend

Now that the API for qemuBuildChrChardevCommand is sane enough, we can
use it to centralize formatting of '-chardev' generally.

For virtiofs we don't have a centrally stored chardev source so we
allocate one inline for temporary use.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>