Adding network testing support for xm-test with hvm.

Adding network testing support for xm-test with hvm.
 1) Added capability for create_disk_image script to copy pcnet32
    driver to disk.img. Added new cmdline option so can specify
    driver directory.
 2) Added configuration --with-driver-dir=DVRDIR option so
    can specify where driver is at configure time. "builtin"
    keyword lets specify that driver is built into kernel.
 3) Updated Makefile.am to build with new options.
 4) Updated README to reflect changes.

Signed-off-by: Daniel Stekloff <dsteklof@us.ibm.com>