VMware balloon: Support 2m page ballooning.

2m ballooning significantly reduces the hypervisor side (and guest side)
overhead of ballooning and unballooning.

hypervisor only:
      balloon  unballoon
4 KB  2 GB/s   2.6 GB/s
2 MB  54 GB/s  767 GB/s

Use 2 MB pages as the hypervisor is alwys 64bit and 2 MB is the smallest
supported super-page size.

The code has to run on older versions of ESX and old balloon drivers run on
newer version of ESX. Hence match the capabilities with the host before 2m
page ballooning could be enabled.

Signed-off-by: Xavier Deguillard <xdeguillard@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>