block: replace unchecked strdup/malloc/calloc with glib

Most of the codebase as been converted to use glib memory allocation
functions.  There are still a few instances of malloc/calloc in the
block layer and qemu-io.  Replace them, especially since they do not
check the strdup/malloc/calloc return value.

Reported-by: Dr David Alan Gilbert <davidagilbert@uk.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
