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-fullvirt-serial-tcp-telnet.cfg

@@ -3,9 +3,6 @@ uuid = "c7a5fdb2-cdaf-9455-926a-d65c16db1809"
 maxmem = 579
 memory = 394
 vcpus = 1
-builder = "hvm"
-kernel = "/usr/lib/xen/boot/hvmloader"
-boot = "d"
 pae = 1
 acpi = 1
 apic = 1
@@ -22,4 +19,7 @@ vncpasswd = "123poi"
 vif = [ "mac=00:16:3e:66:92:9c,bridge=xenbr1,script=vif-bridge,model=e1000,type=ioemu" ]
 parallel = "none"
 serial = "telnet:127.0.0.1:9999,server,nowait"
+builder = "hvm"
+kernel = "/usr/lib/xen/boot/hvmloader"
+boot = "d"
 disk = [ "phy:/dev/HostVG/XenGuest2,hda,w", "file:/root/boot.iso,hdc:cdrom,r" ]