src/xenconfig: move common parsing/formatting to xen_common

src/xenconfig: move common parsing/formatting to xen_common

XM and XL config are very similar.  Disks are specified differently
in XL, but the old XM disk config is still supported by XL.  XL also
supports new config like spice that was never supported by XM.

This patch moves all the common parsing and formatting functions to
the new file xen_common.c and adapts the XM parser/formatter accordingly.
This restructuring paves way for introducing an XL parser/formatter in
the future.

While moving the code, fixup whitespace, comments, and style issues.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>