qemu: command: Use JSON for parameters of -compat

qemu: command: Use JSON for parameters of -compat

'-compat' as a modern implementation based on QAPI already takes JSON as
the argument. Convert our code to use it directly.

QEMU declares the ised QAPI types as:

{ 'enum': 'CompatPolicyInput',
  'data': [ 'accept', 'reject', 'crash' ] }

{ 'enum': 'CompatPolicyOutput',
  'data': [ 'accept', 'hide' ] }

{ 'struct': 'CompatPolicy',
  'data': { '*deprecated-input': 'CompatPolicyInput',
            '*deprecated-output': 'CompatPolicyOutput' } }

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>