qemu-common: Move tcg_enabled() etc. to sysemu/tcg.h

Other accelerators have their own headers: sysemu/hax.h, sysemu/hvf.h,
sysemu/kvm.h, sysemu/whpx.h.  Only tcg_enabled() & friends sit in
qemu-common.h.  This necessitates inclusion of qemu-common.h into
headers, which is against the rules spelled out in qemu-common.h's
file comment.

Move tcg_enabled() & friends into their own header sysemu/tcg.h, and
adjust #include directives.

Cc: Richard Henderson <rth@twiddle.net>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190523143508.25387-2-armbru@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
[Rebased with conflicts resolved automatically, except for
accel/tcg/tcg-all.c]

exec.c

@@ -16,6 +16,7 @@
  * You should have received a copy of the GNU Lesser General Public
  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  */
+
 #include "qemu/osdep.h"
 #include "qapi/error.h"
 
@@ -32,6 +33,7 @@
 #endif
 #include "sysemu/kvm.h"
 #include "sysemu/sysemu.h"
+#include "sysemu/tcg.h"
 #include "qemu/timer.h"
 #include "qemu/config-file.h"
 #include "qemu/error-report.h"