Steps to resize VirtualBox disk - on host side (Windows) and guest size (Linux)
On host side(Windows):
c:\Program Files\Oracle\VirtualBox>vboxmanage modifyhd "c:\Grudev\Virtual Machines\CentOS7_openstack_AIO_VBOX\CentOS_openstack.vdi" --resize 40000
On guest side (Linux)
:fdisk dev/sda
delete partition
create primary partition
### Remember to change the partition type of Linux LVM if needed (command t, hex code=8e). Or you'll be in trouble.
pvresize /dev/sda2
lvextend -l +100%FREE /dev/centos/root or lvextend -L +10G /dev/centos/root
resize2fs (for centos7 and rhel7 try using xfs_growfs) /dev/centos/root