block_resize: Implement qemu driver method

block_resize: Implement qemu driver method

It requires the domain is running, otherwise fails. Resize to a lower
size is supported, but should be used with extreme caution.

In order to prohibit the "size" overflowing after multiplied by
1024. We do checking in the codes. For QMP mode, the default units
is Bytes, the passed size needs to be multiplied by 1024, however,
for HMP mode, the default units is "Megabytes", the passed "size"
needs to be divided by 1024 then.