qtest: split configuration of qtest accelerator and chardev

qtest uses the icount infrastructure to implement a test-driven vm_clock.  This
however is not necessary when using -qtest as a "probe" together with a normal
TCG-, KVM- or Xen-based virtual machine.  Hence, split out the call to
configure_icount into a new function that is called only for "-machine
accel=qtest"; and disable those commands when running with an accelerator
other than qtest.

This also fixes an assertion failure with "qemu-system-x86_64 -machine
accel=qtest" but no -qtest option.  This is a valid case, albeit somewhat
weird; nothing will happen in the VM but you'll still be able to
interact with the monitor or the GUI.

Now that qtest_init is not limited to an int(void) function, change
global variables that are not used outside qtest_init to arguments.

And finally, cleanup useless parts of include/sysemu/qtest.h.  The file
is not used at all for user-only emulation, and qtest is not available
on Win32 due to its usage of sigwait.

Reported-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
