hw/block/fdc: Convert to realize

Convert floppy_drive_init() to realize and rename it to
floppy_drive_realize().

Cc: John Snow <jsnow@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>

Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 87119b34f32e2acf7166165fb5d8e6fca787b3bc.1505737465.git.maozy.fnst@cn.fujitsu.com
Signed-off-by: John Snow <jsnow@redhat.com>