virttest.utils_libguestfs: Add sets of commands for is-*

Add following commands:
is-blockdev
is-blockdev-opts
is-chardev
is-chardev-opts
is-file-opts
is-dir
is-dir-opts
is-socket
is-socket-opts
is-fifo
is-fifo-opts
is-zero
is-whole-device
is-lv
is-symlink
is-zero-device

Modify one:
is-file