hostmem-file: fix pmem file size check

hostmem-file: fix pmem file size check

Commit 314aec4a6e06844937f1677f6cba21981005f389 ("hostmem-file: reject
invalid pmem file sizes") added a file size check that verifies the
hostmem object's size parameter against the actual devdax pmem file.
This is useful because getting the size wrong results in confusing
errors inside the guest.

However, the code doesn't work properly for files where struct
stat::st_size is zero.  Hostmem-file's ->alloc() function returns early
without setting an Error, causing the following assertion failure:

  qemu/memory.c:2215: memory_region_get_ram_ptr: Assertion `mr->ram_block' failed.

This patch handles the case where qemu_get_pmem_size() returns 0 but
there is no error.

Fixes: 314aec4a6e06844937f1677f6cba21981005f389
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20190823135632.25010-1-stefanha@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>