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>