qtest: Move accel code to accel/qtest.c

qtest: Move accel code to accel/qtest.c

QTest has two parts: the server (-qtest) and the accelerator
(-machine accel=qtest).  The accelerator depends on CONFIG_POSIX
due to its usage of sigwait(), but the server doesn't.

Move the accel code to accel/qtest.c.  Later we will disable
compilation of accel/qtest.c on non-POSIX systems.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190422210448.2488-2-ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[thuth: added fixup for MAINTAINERS file]
Signed-off-by: Thomas Huth <thuth@redhat.com>