vl: switch -M parsing to keyval

Switch from QemuOpts to keyval.  This enables the introduction
of non-scalar machine properties, and JSON syntax in the future.

For JSON syntax to be supported right now, we would have to
consider what would happen if string-based dictionaries (produced by
-M key=val) were to be merged with strongly-typed dictionaries
(produced by -M {'key': 123}).

The simplest way out is to never enter the situation, and only allow one
-M option when JSON syntax is in use.  However, we want options such as
-smp to become syntactic sugar for -M, and this is a problem; as soon
as -smp becomes a shortcut for -M, QEMU would forbid using -M '{....}'
together with -smp.  Therefore, allowing JSON syntax right now for -M
would be a forward-compatibility nightmare and it would be impossible
anyway to introduce -M incrementally in tools.

Instead, support for JSON syntax is delayed until after the main
options are converted to QOM compound properties.  These include -boot,
-acpitable, -smbios, -m, -semihosting-config, -rtc and -fw_cfg.  Once JSON
syntax is introduced, these options will _also_ be forbidden together
with -M '{...}'.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>