xen: add CONFIG_XEN_BUS and CONFIG_XEN_EMU options for Xen emulation

xen: add CONFIG_XEN_BUS and CONFIG_XEN_EMU options for Xen emulation

The XEN_EMU option will cover core Xen support in target/, which exists
only for x86 with KVM today but could theoretically also be implemented
on Arm/Aarch64 and with TCG or other accelerators (if anyone wants to
run the gauntlet of struct layout compatibility, errno mapping, and the
rest of that fui).

It will also cover the support for architecture-independent grant table
and event channel support which will be added in hw/i386/kvm/ (on the
basis that the non-KVM support is very theoretical and making it not use
KVM directly seems like gratuitous overengineering at this point).

The XEN_BUS option is for the xenfv platform support, which will now be
used both by XEN_EMU and by real Xen.

The XEN option remains dependent on the Xen runtime libraries, and covers
support for real Xen. Some code which currently resides under CONFIG_XEN
will be moving to CONFIG_XEN_BUS over time as the direct dependencies on
Xen runtime libraries are eliminated. The Xen PCI platform device will
also reside under CONFIG_XEN_BUS.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>