KVM: x86 emulator: clean up control flow in x86_emulate_insn()

x86_emulate_insn() is full of things like

    if (rc != X86EMUL_CONTINUE)
        goto done;
    break;

consolidate all of those at the end of the switch statement.

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