qemu: use correct backendType when checking memfd capability

qemu: use correct backendType when checking memfd capability

The backend name is memory-backend-memfd but we've been checking
for memory-backend-memory.

Reported by GCC on rawhide:
../../../src/internal.h:75:22: error: 'strcmp' of a string of length 21 and
an array of size 21 evaluates to nonzero [-Werror=string-compare]
../../../src/qemu/qemu_command.c:3525:20: note: in expansion of macro 'STREQ'
 3525 |         } else if (STREQ(backendType, "memory-backend-memory") &&
      |                    ^~~~~

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Fixes: 24b74d187cab48a9dc9f409ea78900154c709579
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>