util: Rename variable "major" in virIsDevMapperDevice
major() is a macro defined in sys/sysmacros.h so luckily the code works,
but it's very confusing. Let's rename the local variable to make the
difference between it and the macro more obvious. And while touching the
line we can also initialize it to make sure "clever" analyzers do not
think it may be used uninitialized.
Signed-off-by: Jiri Denemark <jdenemar@redhat.com> Reviewed-by: Laine Stump <laine@redhat.com>