kvm-all: Use 'tmpcpu' instead of 'cpu' in sub-looping to avoid 'cpu' be NULL

If kvm_arch_remove_sw_breakpoint() in CPU_FOREACH() always be fail, it
will let 'cpu' NULL. And the next kvm_arch_remove_sw_breakpoint() in
QTAILQ_FOREACH_SAFE() will get NULL parameter for 'cpu'.

And kvm_arch_remove_sw_breakpoint() can assumes 'cpu' must never be NULL,
so need define additional temporary variable for 'cpu' to avoid the case.

Cc: qemu-stable@nongnu.org
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
