x86/hvm: Drop hvm_{get,set}_guest_bndcfgs() and use {get,set}_regs() instead

x86/hvm: Drop hvm_{get,set}_guest_bndcfgs() and use {get,set}_regs() instead

hvm_{get,set}_guest_bndcfgs() are thin wrappers around accessing MSR_BNDCFGS.

MPX was implemented on Skylake uarch CPUs and dropped in subsequent CPUs, and
is disabled by default in Xen VMs.

It would be nice to move all the logic into vmx_msr_{read,write}_intercept(),
but the common HVM migration code uses guest_{rd,wr}msr().  Therefore, use
{get,set}_regs() to reduce the quantity of "common" HVM code.

In lieu of having hvm_set_guest_bndcfgs() split out, use some #ifdef
CONFIG_HVM in guest_wrmsr().  In vmx_{get,set}_regs(), split the switch
statements into two depending on whether the require remote VMCS acquisition
or not.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>