vl: Fix -drive / -blockdev persistent reservation management

qemu-system-FOO's main() acts on command line arguments in its own
idiosyncratic order.  There's not much method to its madness.
Whenever we find a case where one kind of command line argument needs
to refer to something created for another kind later, we rejigger the
order.

Recent commit cda4aa9 "vl: Create block backends before setting
machine properties" was such a rejigger.  Block backends are now
created before "delayed" objects.  This broke persistent reservation
management.  Reproducer:

    $ qemu-system-x86_64 -object pr-manager-helper,id=pr-helper0,path=/tmp/pr-helper0.sock-drive -drive file=/dev/mapper/crypt,file.pr-manager=pr-helper0,format=raw,if=none,id=drive-scsi0-0-0-2
    qemu-system-x86_64: -drive file=/dev/mapper/crypt,file.pr-manager=pr-helper0,format=raw,if=none,id=drive-scsi0-0-0-2: No persistent reservation manager with id 'pr-helper0'

The delayed pr-manager-helper object is created too late for use by
-drive or -blockdev.  Normal objects are still created in time.

pr-manager-helper has always been a delayed object (commit 7c9e527
"scsi, file-posix: add support for persistent reservation
management").  Turns out there's no real reason for that.  Make it a
normal object.

Fixes: cda4aa9
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190604151251.9903-2-armbru@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

vl.c

@@ -2751,8 +2751,7 @@ static bool object_create_initial(const char *type, QemuOpts *opts)
         exit(0);
     }
 
-    if (g_str_equal(type, "rng-egd") ||
-        g_str_has_prefix(type, "pr-manager-")) {
+    if (g_str_equal(type, "rng-egd")) {
         return false;
     }