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

hw/vfio/pci.c

@@ -939,7 +939,7 @@ static void vfio_pci_load_rom(VFIOPCIDevice *vdev)
     };
     uint64_t size;
     off_t off = 0;
-    size_t bytes;
+    ssize_t bytes;
 
     if (ioctl(vdev->vbasedev.fd, VFIO_DEVICE_GET_REGION_INFO, &reg_info)) {
         error_report("vfio: Error getting ROM info: %m");