CONFIG_XEN_PCI_PASSTHROUGH is just a global configuration option;
it is never used in the source files, so there is no need to put
CONFIG_XEN_PCI_PASSTHROUGH in config-target.h or even in config-host.h.
This inaccuracy was copied over from the configure script in commit
8a19980e3f ("configure: move accelerator logic to meson", 2020-10-03).
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>