KVM: Move halt-polling documentation into common directory

Move halt-polling.rst into the common KVM documentation directory and
out of the x86-specific directory. Halt-polling is a common feature and
the existing documentation is already written as such.
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20221201195249.3369720-2-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/index.rst

@@ -17,4 +17,5 @@ KVM
 
    locking
    vcpu-requests
+   halt-polling
    review-checklist

Documentation/virt/kvm/x86/index.rst

@@ -10,7 +10,6 @@ KVM for x86 systems
    amd-memory-encryption
    cpuid
    errata
-   halt-polling
    hypercalls
    mmu
    msr