Initialize the number of LVT entries during APIC creation, else the field
will be incorrectly left '0' if userspace never invokes KVM_X86_SETUP_MCE.

Add and use a helper to calculate the number of entries even though
MCG_CMCI_P is not set by default in vcpu->arch.mcg_cap.  Relying on that
to always be true is unnecessarily risky, and subtle/confusing as well.

Fixes: 4b903561 ("KVM: x86: Add Corrected Machine Check Interrupt (CMCI) emulation to lapic.")
Reported-by: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: Jue Wang <juew@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>