build: Require glib >= 2.56

build:  Require glib >= 2.56

There are a few uses of g_autoslist in the qemu driver and likely more
will come throughout the codebase in the future. g_autoslist first
appeared in glib 2.56, so bump the minimum version

https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html#g-autoslist

Bumping the minimum version is an opportune time to update the list of
minimum glib versions found on the distros targeted by libvirt's
platform support policy

   RHEL-7: 2.56.1
   RHEL-8: 2.56.4
   Debian (Buster): 2.58.3
   OpenBSD (Ports): 2.66.7
   FreeBSD (Ports): 2.66.7
   openSUSE Leap 15.2, SLE15-SP2: 2.62.6
   Ubuntu (Bionic): 2.56.1
   macOS (Homebrew): 2.66.7

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>