x86/apic: Initialize Secure AVIC APIC backing page

With Secure AVIC, the APIC backing page is owned and managed by the guest.
Allocate and initialize APIC backing page for all guest CPUs.

The NPT entry for a vCPU's APIC backing page must always be present when the
vCPU is running in order for Secure AVIC to function. A VMEXIT_BUSY is
returned on VMRUN and the vCPU cannot be resumed otherwise.

To handle this, notify GPA of the vCPU's APIC backing page to the hypervisor
by using the SVM_VMGEXIT_SECURE_AVIC GHCB protocol event. Before executing
VMRUN, the hypervisor makes use of this information to make sure the APIC
backing page is mapped in the NPT.

  [ bp: Massage commit message. ]

Co-developed-by: Kishon Vijay Abraham I <kvijayab@amd.com>
Signed-off-by: Kishon Vijay Abraham I <kvijayab@amd.com>
Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tianyu Lan <tiala@microsoft.com>
Link: https://lore.kernel.org/20250828070334.208401-3-Neeraj.Upadhyay@amd.com

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