VMware: Remove VMwareImage.file_size_in_gb

file_size_in_gb was rounding down. This is counter intuitive, as a
512MB disk is reported as 0GB. However, this rounding weirdness was
required for correctness in a way which wasn't obvious. When we test
if an image needs to be resized, we test instance.root_gb >
image.file_size_in_gb. Because file_size_in_gb was rounded down, an
image slightly smaller than 1GB will be reported as 0GB and therefore
resized.

The source of the rounding confusion is the bytes->gb conversion. To
make the code more obviously correct, I have switched it to instead
convert gb->bytes, which does not involve rounding. To ensure this
never trips anybody up, I have removed file_size_in_gb.

Change-Id: I99d20e3b1383e29cddff3d1016c4bcedc1683c4d
Merge "VMware: Remove VMwareImage.file_size_in_gb"