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>