xen: introduce XENFEAT_direct_mapped and XENFEAT_not_direct_mapped

xen: introduce XENFEAT_direct_mapped and XENFEAT_not_direct_mapped

Introduce two feature flags to tell the domain whether it is
direct-mapped or not. It allows the guest kernel to make informed
decisions on things such as swiotlb-xen enablement.

The introduction of both flags (XENFEAT_direct_mapped and
XENFEAT_not_direct_mapped) allows the guest kernel to avoid any
guesswork if one of the two is present, or fallback to the current
checks if neither of them is present.

XENFEAT_direct_mapped is always set for not auto-translated guests.

For auto-translated guests, only Dom0 on ARM is direct-mapped. Also,
see is_domain_direct_mapped() which refers to auto-translated guests:
xen/include/asm-arm/domain.h:is_domain_direct_mapped
xen/include/asm-x86/domain.h:is_domain_direct_mapped

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
CC: jbeulich@suse.com
CC: andrew.cooper3@citrix.com
CC: julien@xen.org