domain_conf: move duplicate check functions to domain_validate.c

domain_conf: move duplicate check functions to domain_validate.c

virDomainDefCheckDuplicateDiskInfo() and virDomainDefCheckDuplicateDriveAddresses()
are static functions used by virDomainDefValidateInternal(). Let's
move them to domain_validate.c to start clearing up the path to
move virDomainDefValidateInternal().

Change the functions name slightly to be more on par with their
new home.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>