require newer glib2 to enable autofree'ing of stack variables exiting scope

* Bump minium glib2 version to 2.48
* Convert much of the crypto code to use automatic memory free functions

# gpg: Signature made Thu 22 Aug 2019 11:51:59 BST
# gpg:                using RSA key DAF3A6FDB26B62912D0E8E3FBE86EBB415104FDF
# gpg: Good signature from "Daniel P. Berrange <dan@berrange.com>" [full]
# gpg:                 aka "Daniel P. Berrange <berrange@redhat.com>" [full]
# Primary key fingerprint: DAF3 A6FD B26B 6291 2D0E  8E3F BE86 EBB4 1510 4FDF

* remotes/berrange/tags/autofree-pull-request:
  crypto: use auto cleanup for many stack variables
  crypto: define cleanup functions for use with g_autoptr
  glib: bump min required glib library version to 2.48

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>