Update documentation and scripts to reflect changes in xfsMonkey.py for btrfs

[drholmie]

Currently the README.md, vm_remote_trigger_script.sh and demo.sh use 100MB as the specified size when running xfsMoneky.py with file system as btrfs, even thought it is mentioned in xfsMonkey.py (here: https://github.com/utsaslab/crashmonkey/blob/master/xfsMonkey.py#L84) that btrfs requires at least 200MB.

This change occurred in the following commit: https://github.com/utsaslab/crashmonkey/commit/5ede8f37b299da3f3c99ca33837948255efc62f3#diff-cd90afe905f7f1227973434f35165b3c. It appears that updating the README.md, vm_remote_trigger_script.sh and demo.sh appropriately was accidentally overlooked.