KVM: x86: Fold "struct dest_map" into "struct rtc_status"

KVM: x86: Fold "struct dest_map" into "struct rtc_status"

Drop "struct dest_map" and fold its members into its one and only user,
"struct rtc_status".  Tracking "pending" EOIs and associated vCPUs is very
much a hack for legacy RTC behavior, and should never be needed for other
IRQ delivery.  In addition to making it more obvious why KVM tracks target
vCPUs, this will allow burying the "struct rtc_status" definition behind
CONFIG_KVM_IOAPIC=y, which in turn will make it even harder for KVM to
misuse the structure.

No functional change intended.

Link: https://patch.msgid.link/20251206004311.479939-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>