xen: fix usage of ENODATA

xen: fix usage of ENODATA

ENODATA doesn't exist on FreeBSD, so ENODATA errors returned by the
hypervisor are translated to ENOENT.

Also, the error code is returned in errno if the call returns -1, so
compare the error code with the value in errno instead of the value
returned by the function.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: xen-devel@lists.xenproject.org
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Anthony Perard <anthony.perard@citrix.com>