Do not drop kernel cmdline for xen pv domains

Kernel cmdline args can be passed to xen pv domains even when a
bootloader is specified.  The current config-to-sxpr mapping
ignores cmdline when bootloader is present.

Since the xend sub-driver is used with many xen toolstack versions,
this patch takes conservative approach of adding an else block to
existing !def->os.bootloader, and only appends sxpr if def->os.cmdline
is non-NULL.

V2: Fix existing testcase broken by this patch and add new testcases

tests/xml2sexprdata/xml2sexpr-disk-block-shareable.sexpr

@@ -1,6 +1,7 @@
 (vm (name 'pvtest')(memory 384)(maxmem 512)(vcpus 1)\
 (uuid '49a0c6ff-c066-5392-6498-3632d093c2e7')(bootloader '/usr/bin/pygrub')\
 (on_poweroff 'destroy')(on_reboot 'restart')(on_crash 'restart')\
+(image (linux (args 'ro root=/dev/VolGroup00/LogVol00')))\
 (device (tap (dev 'xvda')(uname 'tap:aio:/var/lib/xen/images/rhel5pv.img')\
 (mode 'w!')))(device (vif (mac '00:16:3e:23:9e:eb')(bridge 'xenbr0')\
 (script 'vif-bridge'))))\

tests/xml2sexprdata/xml2sexpr-pv-bootloader-cmdline.sexpr

@@ -0,0 +1,5 @@
+(vm (name 'pvtest')(memory 420)(maxmem 420)(vcpus 2)\
+(uuid '596a5d21-71f4-8fb2-e068-e2386a5c413e')(bootloader '/usr/bin/pygrub')\
+(bootloader_args '-q')(on_poweroff 'destroy')(on_reboot 'destroy')\
+(on_crash 'destroy')(image (linux (args 'xenfb.video=8,1280,1024')))\
+(device (vbd (dev 'xvda')(uname 'file:/root/some.img')(mode 'w'))))\

tests/xml2sexprdata/xml2sexpr-pv-bootloader-cmdline.xml

@@ -0,0 +1,22 @@
+<domain type='xen' id='15'>
+  <name>pvtest</name>
+  <uuid>596a5d2171f48fb2e068e2386a5c413e</uuid>
+  <bootloader>/usr/bin/pygrub</bootloader>
+  <bootloader_args>-q</bootloader_args>
+ <os>
+    <type>linux</type>
+    <cmdline>xenfb.video=8,1280,1024</cmdline>
+  </os>
+  <memory>430080</memory>
+  <vcpu>2</vcpu>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>destroy</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <disk type='file' device='disk'>
+      <source file='/root/some.img'/>
+      <target dev='xvda'/>
+    </disk>
+    <console tty='/dev/pts/4'/>
+  </devices>
+</domain>