xenconfig: don't use "kernel" for hvmloader

xenconfig: don't use "kernel" for hvmloader

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.