x86/apic: Add support to send IPI for Secure AVIC

Secure AVIC hardware accelerates only Self-IPI, i.e. on WRMSR to APIC_SELF_IPI
and APIC_ICR (with destination shorthand equal to Self) registers, hardware
takes care of updating the APIC_IRR in the APIC backing page of the vCPU.

For other IPI types (cross-vCPU, broadcast IPIs), software needs to take care
of updating the APIC_IRR state of the target vCPUs and to ensure that the
target vCPUs notice the new pending interrupt.

Add new callbacks in the Secure AVIC driver for sending IPI requests.  These
callbacks update the IRR in the target guest vCPU's APIC backing page. To
ensure that the remote vCPU notices the new pending interrupt, reuse the GHCB
MSR handling code in vc_handle_msr() to issue APIC_ICR MSR-write GHCB protocol
event to the hypervisor.

For Secure AVIC guests, on APIC_ICR write MSR exits, the hypervisor notifies
the target vCPU by either sending an AVIC doorbell (if target vCPU is running)
or by waking up the non-running target vCPU.

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/20250828110824.208851-1-Neeraj.Upadhyay@amd.com

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