qapi-types: add C99 index names to arrays

It's not easy to figure out how monitor translates
strings: most QEMU code deals with translated indexes,
these are translated using _lookup arrays,
so you need to find the array name, and find the
appropriate offset.

This patch adds C99 indexes to lookup arrays, which makes it possible to
find the correct key using simple grep, and see that the matching is
correct at a glance.

Example:

Before:

const char *MigrationCapability_lookup[] = {
    "xbzrle",
    "rdma-pin-all",
    "auto-converge",
    "zero-blocks",
    NULL,
};

After:

const char *MigrationCapability_lookup[] = {
    [MIGRATION_CAPABILITY_XBZRLE] = "xbzrle",
    [MIGRATION_CAPABILITY_RDMA_PIN_ALL] = "rdma-pin-all",
    [MIGRATION_CAPABILITY_AUTO_CONVERGE] = "auto-converge",
    [MIGRATION_CAPABILITY_ZERO_BLOCKS] = "zero-blocks",
    [MIGRATION_CAPABILITY_MAX] = NULL,
};

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>