x86: stop handling MSR_IA32_BNDCFGS save/restore in implementation code

x86: stop handling MSR_IA32_BNDCFGS save/restore in implementation code

Saving and restoring the value of this MSR is currently handled by
implementation-specific code despite it being architectural. This patch
moves handling of accesses to this MSR from hvm.c into the msr.c, thus
allowing the common MSR save/restore code to handle it.

NOTE: Because vmx_get/set_guest_bndcfgs() call vmx_vmcs_enter(), the
      struct vcpu pointer passed in, and hence the vcpu pointer passed to
      guest_rdmsr() cannot be const.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>