Differentiate 'distro' and 'distrofamily' in Sys::Guestfs::Lib

Differentiate 'distro' and 'distrofamily' in Sys::Guestfs::Lib

Change distro in the output formally known as virt-inspector to reflect the
actual distro. Possible values are now: fedora, rhel, centos, scientific,
debian.

Add new distrofamily entry which is one of: redhat, debian. Currently all
distros except 'debian' are in the redhat family.

This allows you to, for example, select a RHEL/CentOS/Scientific Linux specific
kernel for installation rather than assuming they're all the same.

Note this also changes the behaviour of virt-inspector --query. It will now only
return rhel=yes for RHEL.