KVM: x86 emulator: rename decode_cache::eip to _eip

The name eip conflicts with a field of the same name in x86_emulate_ctxt,
which we plan to fold decode_cache into.

The name _eip is unfortunate, but what's really needed is a refactoring
here, not a better name.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>