xapi_base.rb: fix get_sr_by_name()

fix get_sr_by_name() as xapi.SR.get_by_name_label returns an array.
xapi_base.rb: fix print_vdi_info()

remove usage of deprecated and now removed #bytes method
of the Integer class.
Merge pull request #9 from lebrinkma/fix-vdi

Fix VDI Subcommands resolves #8