glib: bump min required glib library version to 2.48

glib: bump min required glib library version to 2.48

Per supported platforms doc[1], the various min glib on relevant distros is:

  RHEL-8: 2.56.1
  RHEL-7: 2.50.3
  Debian (Buster): 2.58.3
  Debian (Stretch): 2.50.3
  OpenBSD (Ports): 2.58.3
  FreeBSD (Ports): 2.56.3
  OpenSUSE Leap 15: 2.54.3
  SLE12-SP2: 2.48.2
  Ubuntu (Xenial): 2.48.0
  macOS (Homebrew): 2.56.0

This suggests that a minimum glib of 2.48 is a reasonable target.

Compared to the previous version bump in

  commit e7b3af81597db1a6b55f2c15d030d703c6b2c6ac
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Fri May 4 15:34:46 2018 +0100

    glib: bump min required glib library version to 2.40

This will result in us dropping support for Debian Jessie and
Ubuntu 14.04.

As per the commit message 14.04 was already outside our list
of supported build platforms and an exception was only made
because one of the build hosts used during merge testing was
stuck on 14.04.

Debian Jessie is justified to drop because we only aim to
support at most 2 major versions of Debian at any time. This
means Buster and Stretch at this time.

The g_strv_contains compat code is dropped as this API is
present since 2.44

The g_assert_cmpmem compat code is dropped as this API is
present since 2.46

[1] https://qemu.weilnetz.de/doc/qemu-doc.html#Supported-build-platforms

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>