KVM: VMX: make MSR bitmaps per-VCPU

Place the MSR bitmap in struct loaded_vmcs, and update it in place
every time the x2apic or APICv state can change.  This is rare and
the loop can handle 64 MSRs per iteration, in a similar fashion as
nested_vmx_prepare_msr_bitmap.

This prepares for choosing, on a per-VM basis, whether to intercept
the SPEC_CTRL and PRED_CMD MSRs.

Cc: [email protected]       # prereq for Spectre mitigation
Suggested-by: Jim Mattson <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: bonzini