KVM: x86: Move "kvm_rebooting" to kernel as "virt_rebooting"

KVM: x86: Move "kvm_rebooting" to kernel as "virt_rebooting"

Move "kvm_rebooting" to the kernel, exported for KVM, as one of many steps
towards extracting the innermost VMXON and EFER.SVME management logic out
of KVM and into to core x86.

For lack of a better name, call the new file "hw.c", to yield "virt
hardware" when combined with its parent directory.

No functional change intended.

Tested-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Sagi Shahar <sagis@google.com>
Link: https://patch.msgid.link/20260214012702.2368778-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>