physmem: fix qemu_ram_alloc_from_fd size calculation

cheery-pick from 719168fba7c3215cc996dcfd32a6e5e9c7b8eee0

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>

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