KVM: x86: Add a helper to dedup loading guest/host XCR0 and XSS

KVM: x86: Add a helper to dedup loading guest/host XCR0 and XSS

Add and use a helper, kvm_load_xfeatures(), to dedup the code that loads
guest/host xfeatures.

Opportunistically return early if X86_CR4_OSXSAVE is not set to reduce
indentations.

No functional change intended.

Suggested-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://patch.msgid.link/20251110050539.3398759-1-binbin.wu@linux.intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>