hostmem: Apply merge property after the memory region is initialized

JIRA: https://issues.redhat.com/browse/RHEL-68289
Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=65965311
Upstream Status: QEMU v9.2.0
Tested: Passed, no core dump observed

The "merge" property of memory backend can be applied, even when its
memory region isn't initialized. A core dump is seen when the "merge"
property is requested to memory backend by user in this scenario.
This picks up the upstream commit 78c8f780d3f0 ("hostmem: Apply merge
property after the memory region is initialized") to fix the issue.

Signed-off-by: Gavin Shan gshan@redhat.com