gdbstub: Fix vCont behaviour

When GDB issues a "vCont", QEMU was not handling it correctly when
multiple VCPUs are active.
For vCont, for each thread (VCPU), it can be specified whether to
single step, continue or stop that thread. The default is to stop a
thread.
However, when (for example) "vCont;s:2" is issued, all VCPUs continue
to run, although all but VCPU nr 2 are to be stopped.

This patch completely rewrites the vCont parsing code.

Please note that this improvement only works in system emulation mode,
when in userspace emulation mode the old behaviour is preserved.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Message-Id: <1487092068-16562-3-git-send-email-imbrenda@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>