physmem: fix qemu_ram_alloc_from_fd size calculation

physmem: fix qemu_ram_alloc_from_fd size calculation

qemu_ram_alloc_from_fd allocates space if file_size == 0.  If non-zero,
it uses the existing space and verifies it is large enough, but the
verification was broken when the offset parameter was introduced.  As
a result, a file smaller than offset passes the verification and causes
errors later.  Fix that, and update the error message to include offset.

Peter provides this concise reproducer:

  $ touch ramfile
  $ truncate -s 64M ramfile
  $ ./qemu-system-x86_64 -object memory-backend-file,mem-path=./ramfile,offset=128M,size=128M,id=mem1,prealloc=on
  qemu-system-x86_64: qemu_prealloc_mem: preallocating memory failed: Bad address

With the fix, the error message is:
  qemu-system-x86_64: mem1 backing store size 0x4000000 is too small for 'size' option 0x8000000 plus 'offset' option 0x8000000

Cc: qemu-stable@nongnu.org
Fixes: 4b870dc4d0c0 ("hostmem-file: add offset option")
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/1736967650-129648-3-git-send-email-steven.sistare@oracle.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>