xenconfig: support bios=ovmf xl.cfg

Add support to xenconfig for conversion of xl.cfg(5) bios config
to/from libvirt domXml <loader> config. SeaBIOS is the default
for HVM guests using upstream QEMU. ROMBIOS is the default when
using the old qemu-dm. This patch allows specifying OVMF as an
alternate firmware.

Example xl.cfg:
  bios = "ovmf"

Example domXML:
  <os>
    ...
    <loader readonly='yes' type='pflash'>/usr/lib/xen/boot/ovmf.bin</loader>
  </os>

Note that currently Xen does not support a separate nvram for
non-volatile variables.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>

tests/testutils.h

@@ -75,6 +75,8 @@ int virTestDifferenceBin(FILE *stream,
                          size_t length);
 int virTestCompareToFile(const char *strcontent,
                          const char *filename);
+int virTestCompareToString(const char *strcontent,
+                           const char *strsrc);
 
 unsigned int virTestGetDebug(void);
 unsigned int virTestGetVerbose(void);