conf: Always use 'string()' conversion with virXPath(U)LongLong

conf: Always use 'string()' conversion with virXPath(U)LongLong

When the 'string()' conversion is used the number is parsed inside
libvirt by our internal helpers which work on integers in contrast to
when 'number()' is used and libxml2 uses a 'double' variable internally.

On the upper extremes of the 64 bit variables the double precision
variable doesn't have enough precision to represent each distinct
integer and thus could cause problems.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>