x86/apic: Add an update_vector() callback for Secure AVIC

Add an update_vector() callback to set/clear the ALLOWED_IRR field in a vCPU's
APIC backing page for vectors which are emulated by the hypervisor.

The ALLOWED_IRR field indicates the interrupt vectors which the guest allows
the hypervisor to inject (typically for emulated devices).  Interrupt vectors
used exclusively by the guest itself and the vectors which are not emulated by
the hypervisor, such as IPI vectors, should not be set by the guest in the
ALLOWED_IRR fields.

As clearing/setting state of a vector will also be used in subsequent commits
for other APIC registers (such as APIC_IRR update for sending IPI), add
a common update_vector() in the Secure AVIC driver.

  [ 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/20250828110255.208779-4-Neeraj.Upadhyay@amd.com

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