xen/pt: Fix incomplete conversion to realize()

xen/pt: Fix incomplete conversion to realize()

The conversion of "xen-pci-passthrough" to realize() (commit
5a11d0f7549, v2.6.0) neglected to convert the xen_pt_config_init()
error path.  If xen_pt_config_init() fails, xen_pt_realize() reports
the error, then returns success without completing its job.  I don't
know the exact impact, but it can't be good.

Belatedly convert the error path.

Fixes: 5a11d0f7549e24a10e178a9dc8ff5e698031d9a6
Cc: Stefano Stabellini <[email protected]>
Cc: Anthony Perard <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Acked-by: Anthony PERARD <[email protected]>
Message-Id: <20181017082702[email protected]>