KVM: SVM: Refactor AVIC vcpu initialization into avic_init_vcpu()

Preparing the base code for subsequent changes. This does not change
existing logic.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>