x86/vmx: Remove vmx_save_host_msrs() and host_msr_state

x86/vmx: Remove vmx_save_host_msrs() and host_msr_state

A pcpu's LSTAR, STAR and SYSCALL_MASK MSRs are unconditionally switched when
moving in and out of HVM vcpu context.  Two of these values are compile time
constants, and the third is directly available in an existing per-cpu
variable.

There is no need to save host state in vmx_cpu_up() into a different per-cpu
structure, so drop all the infrastructure.  vmx_restore_host_msrs() is
simplified to 3 plain WRMSR instructions.

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