Update FUEL virtualbox environment README file

[Kyrylo Romanenko]

README.md file (https://github.com/stackforge/fuel-main/blob/master/virtualbox/README.md) should be updated to reflect important requirement.

Currently listed requirements:
- VirtualBox with VirtualBox Extension Pack
- procps
- expect
- Cygwin for Windows host PC

Following requirement should be added:
- Enable VT-x/AMD-V acceleration options on your hardware for 64-bits guests

Impact: unscheduled host system reboot to enable VT-x/AMD-V in BIOS if it was not set previously.

If VT-x/AMD-V is not enabled on host system, then VirtualBox generates an error message:
"VT-x/AMD-V hardware acceleration is not available on your system. Your 64-bit guest will fail to detect a 64-bit CPU and will not be able to boot."
VT-x or AMD-V support is needed for 64-bits guests.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/147509

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/147509
Committed: https://git.openstack.org/cgit/stackforge/fuel-main/commit/?id=f34871e95935aa1eca0e01f22a05a922ead21dab
Submitter: Jenkins
Branch: master

commit f34871e95935aa1eca0e01f22a05a922ead21dab
Author: Irina Povolotskaya <email address hidden>
Date: Thu Jan 15 16:50:33 2015 +0300

    Adds a requirement to README.md file on VirtualBox

    Since VT-x/AMD-V acceleration option is
    a requirement, let's add it into the list.

    Change-Id: I6933b117adbccc374eb5b3bc397508787a953907
    Closes-Bug: 1411202