domain_conf: move net device validation to domain_validate.c

domain_conf: move net device validation to domain_validate.c

The next objective is to move virDomainDeviceDefValidate() to
domain_validate.c. First let's move all the static helpers.

The net device validation functions are used across multiple
drivers, so let's move them separately first.

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