vfio: fix return type of pread

size_t is an unsigned type, thus the error case is never reached in
the below call to pread.  If bytes is negative, it will be seen as
a very high positive value.

Spotted by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
