Support new OSv zfs-builder

• compose: handle --fs option to support both ZFS and ROFS
• Support building images with zfs-builder
• Update qemu command line arguments
• qemu: make vnc parameter specify unique UNIX domain socket

Upgrade to new Go modules

For details please see comments in this commit - bf6c68a

Compose app images without yaml, support Hyperkit and Intel HAXM

• QEMU: Added HAXM acceleration for Mac OSX
• Supports running OSv on Docker's Hyperkit on Mac
• Supports rapidly composing apps without a need of explicit YAML manifest files by using a default manifest and new --require option added to package compose
• Enhanced following commands:
  • package describe: added option "-c" to list the content of a specified package
  • package compose: add loader_image option to allow specifying arbitrary OSv kernel
• Made github unit tests more reliable and way faster by using mock
• Revised documentation

Pull OSv kernel and packages from GitHub

• Added ability to pull OSv kernel and packages from OSv GitHub releases repository (please see this for details)
• Made detection of KVM acceleration more portable across all Linux distributions
• The verbose flag (-v, --verbose) when running on QEMU prints full command line with all parameters