KVM: SVM: Move x2AVIC MSR interception helper to avic.c

KVM: SVM: Move x2AVIC MSR interception helper to avic.c

Move svm_set_x2apic_msr_interception() to avic.c as it's only relevant
when x2AVIC is enabled/supported and only called by AVIC code.  In
addition to scoping AVIC code to avic.c, this will allow burying the
global x2avic_enabled variable in avic.

Opportunistically rename the helper to explicitly scope it to "avic".

No functional change intended.

Reviewed-by: Naveen N Rao (AMD) <naveen@kernel.org>
Tested-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/20250919215934.1590410-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>