x86/hvm: Drop get_shadow_gs_base() hook and use hvm_get_reg()

x86/hvm: Drop get_shadow_gs_base() hook and use hvm_get_reg()

This is a trivial accessor for an MSR, so use hvm_get_reg() rather than a
dedicated hook.  In arch_get_info_guest(), rework the logic to read GS_SHADOW
only once.

get_hvm_registers() is called on current, meaning that diagnostics print a
stale GS_SHADOW from the previous vcpu context switch.  Adjust both
implementations to obtain the correct value.

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