bhyve: cdrom support

Add support for CDROM devices for bhyve driver using
bhyve(8)'s 'ahci-cd' device type.

As bhyve currently does not support media insertion at runtime,
disallow to start a domain with an empty source path for cdrom
devices.

tests/bhyvexml2argvdata/bhyvexml2argv-disk-cdrom.args

@@ -0,0 +1,3 @@
+/usr/sbin/bhyve -c 1 -m 214 -H -P -s 0:0,hostbridge \
+-s 3:0,virtio-net,faketapdev,mac=52:54:00:00:00:00 \
+-s 2:0,ahci-cd,/tmp/cdrom.iso bhyve

tests/bhyvexml2argvdata/bhyvexml2argv-disk-cdrom.xml

@@ -0,0 +1,22 @@
+<domain type='bhyve'>
+  <name>bhyve</name>
+  <uuid>df3be7e7-a104-11e3-aeb0-50e5492bd3dc</uuid>
+  <memory>219136</memory>
+  <vcpu>1</vcpu>
+  <os>
+    <type>hvm</type>
+  </os>
+  <devices>
+    <disk type='file' device='cdrom'>
+      <driver name='file' type='raw'/>
+      <source file='/tmp/cdrom.iso'/>
+      <target dev='hdc' bus='sata'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
+    </disk>
+    <interface type='bridge'>
+      <model type='virtio'/>
+      <source bridge="virbr0"/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
+    </interface>
+  </devices>
+</domain>