xenconfig: move <os> parsing/formating to config-specific files

xl and xm differ a bit in how <os> configuration is represented.
E.g. xl config supports <os><nvram .../></os> via its "bios"
setting.

Move the xenParseOS and xenFormatOS functions from xen_common.c
and copy to xen_xl.c and xen_xm.c so they can be customized for
xm vs xl config.  An unfortunate fallout is reordering of entries
in the test config files.

tests/xmconfigdata/test-paravirt-new-pvfb-vncdisplay.cfg

@@ -3,11 +3,11 @@ uuid = "c7a5fdb0-cdaf-9455-926a-d65c16db1809"
 maxmem = 579
 memory = 394
 vcpus = 1
-bootloader = "/usr/bin/pygrub"
 localtime = 0
 on_poweroff = "destroy"
 on_reboot = "restart"
 on_crash = "restart"
 vfb = [ "type=vnc,vncunused=0,vncdisplay=25,vnclisten=127.0.0.1,vncpasswd=123poi" ]
 vif = [ "mac=00:16:3e:66:94:9c,bridge=br0,script=vif-bridge" ]
+bootloader = "/usr/bin/pygrub"
 disk = [ "phy:/dev/HostVG/XenGuest1,xvda,w" ]