xen: add CONFIG_XENFV_MACHINE 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. It will also cover
the support for architecture-independent grant table and event channel
support which will be added in hw/xen/.
The XENFV_MACHINE 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_XENFV_MACHINE over time.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>