Add support for SD cards in nodedev driver

Add support for SD cards in nodedev driver

The nodedev driver currently only detects harddisk, cdrom
and floppy devices. This adds support for SD cards, which
are common storage for ARM devices, eg the Google ChromeBook

<device>
  <name>block_mmcblk0_0xb1c7c08b</name>
  <parent>computer</parent>
  <capability type='storage'>
    <block>/dev/mmcblk0</block>
    <drive_type>sd</drive_type>
    <serial>0xb1c7c08b</serial>
    <size>15758000128</size>
    <logical_block_size>512</logical_block_size>
    <num_blocks>30777344</num_blocks>
  </capability>
</device>

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>