exec: Fix file_ram_alloc() error API violations

exec: Fix file_ram_alloc() error API violations

When os_mem_prealloc() fails, file_ram_alloc() calls qemu_ram_munmap()
and returns null.  Except it doesn't when its @errp argument is null,
because it checks for failure with (errp && *errp).  Introduced in
commit 056b68af77 "fix qemu exit on memory hotplug when allocation
fails at prealloc time".

No caller actually passes null.

Fix anyway: splice in a local Error *err, and error_propagate().

Cc: Igor Mammedov <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Igor Mammedov <[email protected]>
Message-Id: <20191204093625[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>