PCI: only build xen-pcifront in PV-enabled environments

PCI: only build xen-pcifront in PV-enabled environments

The driver's module init function, pcifront_init(), invokes
xen_pv_domain() first thing. That construct produces constant "false"
when !CONFIG_XEN_PV. Hence there's no point building the driver in
non-PV configurations.

Drop the (now implicit and generally wrong) X86 dependency: At present,
XEN_PV can only be set when X86 is also enabled. In general an
architecture supporting Xen PV (and PCI) would want to have this driver
built.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/3a7f6c9b-215d-b593-8056-b5fe605dafd7@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>