pci-assign: Move "Invalid ROM" error message to pci-assign-load-rom.c

In function pci_assign_dev_load_option_rom, For those pci devices don't
have 'rom' file under sysfs or if loading ROM from external file, The
function returns NULL, and won't set the passed 'size' variable.

In these 2 cases, qemu still reports "Invalid ROM" error message, Users
may be confused by it.

Signed-off-by: Lin Ma <lma@suse.com>
Message-Id: <1466010327-22368-1-git-send-email-lma@suse.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
