vl: configure accelerators from -accel options

Drop the "accel" property from MachineState, and instead desugar
"-machine accel=" to a list of "-accel" options.

This has a semantic change due to removing merge_lists from -accel.
For example:

- "-accel kvm -accel tcg" all but ignored "-accel kvm".  This is a bugfix.

- "-accel kvm -accel thread=single" ignored "thread=single", since it
  applied the option to KVM.  Now it fails due to not specifying the
  accelerator on "-accel thread=single".

- "-accel tcg -accel thread=single" chose single-threaded TCG, while now
  it will fail due to not specifying the accelerator on "-accel
  thread=single".

Also, "-machine accel" and "-accel" become incompatible.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/tests/pxe-test.c b/tests/pxe-test.c
index aaae54f..f68d0aa 100644
--- a/tests/pxe-test.c
+++ b/tests/pxe-test.c
@@ -74,7 +74,7 @@
     }
 
     args = g_strdup_printf(
-        "-machine %s,accel=kvm:tcg -nodefaults -boot order=n "
+        "-accel kvm -accel tcg -machine %s -nodefaults -boot order=n "
         "-netdev user,id=" NETNAME ",tftp=./,bootfile=%s,ipv4=%s,ipv6=%s "
         "-device %s,bootindex=1,netdev=" NETNAME " %s",
         test->machine, disk, ipv6 ? "off" : "on", ipv6 ? "on" : "off",