x86/hvm: Improve hvm_efer_valid() using named features

x86/hvm: Improve hvm_efer_valid() using named features

Pick the appropriate cpuid_policy object rather than using hvm_cpuid() or
boot_cpu_data.  This breaks the dependency on current.

As data is read straight out of cpuid_policy, there is no need to work around
the fact that X86_FEATURE_SYSCALL might be clear because of the dynamic
adjustment in hvm_cpuid().  This simplifies the SCE handling, as EFER.SCE can
be set in isolation in 32bit mode on Intel hardware.

Alter nestedhvm_enabled() to be const-correct, allowing hvm_efer_valid() to be
properly const-correct.

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