file_ram_alloc: unify mem-path,mem-prealloc error handling

-mem-prealloc asks to preallocate memory residing on -mem-path path.

Currently QEMU exits in case:

- Memory file has been created but allocation via explicit write
fails.

And it fallbacks to malloc in case:
- Querying huge page size fails.
- Lack of sync MMU support.
- Open fails.
- mmap fails.

Have the same behaviour for all cases: fail in case -mem-path and
-mem-prealloc are specified for regions where the requested size is
suitable for hugepages.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>