vl: Parse legacy default_machine_opts

vl: Parse legacy default_machine_opts

qemu can't start a xen vm after commit d8fb7d0969d5
"vl: switch -M parsing to keyval" with:

$ ./qemu-system-i386 -M xenfv
Unexpected error in object_property_find_err() at ../qom/object.c:1298:
qemu-system-i386: Property 'xenfv-3.1-machine.accel' not found
Aborted (core dumped)

The default_machine_opts handling doesn't process the legacy machine
options like "accel".  Call qemu_apply_legacy_machine_options to provide
the legacy handling.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Message-Id: <20210713021552.19110-1-jandryuk@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>