util: fix locale problem with virStrToDouble().

util: fix locale problem with virStrToDouble().

This commit fixes a locale problem with locales that use comma as a mantissa
separator. Example: 12.34 en_US = 12,34 pt_BR. Since strtod() is a non-safe
function, virStrToDouble() will have problems to parse double numbers from
kernel settings and other double numbers from static files (XMLs, JSONs, etc).

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1457634
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1457481

Signed-off-by: Julio Faracco <jcfaracco@gmail.com>