KVM: x86: Add a switch_db_regs flag to handle TDX's auto-switched behavior

Add a flag KVM_DEBUGREG_AUTO_SWITCH to skip saving/restoring guest
DRs.

TDX-SEAM unconditionally saves/restores guest DRs on TD exit/enter,
and resets DRs to architectural INIT state on TD exit.  Use the new
flag KVM_DEBUGREG_AUTO_SWITCH to indicate that KVM doesn't need to
save/restore guest DRs.  KVM still needs to restore host DRs after TD
exit if there are active breakpoints in the host, which is covered by
the existing code.

MOV-DR exiting is always cleared for TDX guests, so the handler for DR
access is never called, and KVM_DEBUGREG_WONT_EXIT is never set.  Add
a warning if both KVM_DEBUGREG_WONT_EXIT and KVM_DEBUGREG_AUTO_SWITCH
are set.

Opportunistically convert the KVM_DEBUGREG_* definitions to use BIT().

Reported-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Co-developed-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
[binbin: rework changelog]
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Message-ID: <20241210004946.3718496-2-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20250129095902.16391-13-adrian.hunter@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>