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-pci-devs.cfg

@@ -3,9 +3,6 @@ uuid = "cc2315e7-d26a-307a-438c-6d188ec4c09c"
 maxmem = 382
 memory = 350
 vcpus = 1
-builder = "hvm"
-kernel = "/usr/lib/xen/boot/hvmloader"
-boot = "c"
 pae = 1
 acpi = 1
 apic = 1
@@ -21,4 +18,7 @@ vif = [ "mac=00:16:3e:0a:7b:39,bridge=xenbr0,script=vif-bridge,model=e1000,type=
 pci = [ "0001:0c:1b.2", "0000:01:13.0" ]
 parallel = "none"
 serial = "pty"
+builder = "hvm"
+kernel = "/usr/lib/xen/boot/hvmloader"
+boot = "c"
 disk = [ "phy:/dev/sda8,hda,w", ",hdc:cdrom,r" ]