In xl config, hvmloader is implied for hvm guests. It is not
specified with the "kernel" option like xm config. The "kernel"
option, along with "ramdisk" and "extra", is used for HVM direct
kernel boot. Instead of using "kernel" option to populate
virDomainDef object's os.loader->path, use hvmloader discovered
when gathering capabilities.
This change required fixing initialization of capabilities in
the test utils and removing 'kernel = "/usr/lib/xen/boot/hvmloader"'
from the test config 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.