scsi-generic: fix sign extension of READ CAPACITY(10) data

Issuing the READ CAPACITY(10) command in the guest will cause QEMU
to update its knowledge of the maximum accessible LBA in the disk.
The recorded maximum LBA will be wrong if the disk is bigger than
1TB, because ldl_be_p returns a signed int.

When this is fixed, a latent bug will be unmasked.  If the READ
CAPACITY(10) command reported an overflow (0xFFFFFFFF), we must
not overwrite the previously-known maximum accessible LBA, or the guest
will fail to access the disk above the first 2TB.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
scsi-generic: check the return value of bdrv_aio_ioctl in execute_command

This fixes the bug introduced by this commit ad54ae80c73f.
The bdrv_aio_ioctl() still could return null and we should return an error
in that case.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>