configure: use a platform-neutral prefix

configure: use a platform-neutral prefix

Now that the installation is relocatable, there is no need to compile a
Windows-format prefix into Win32 binaries.  Instead, the prefix will
only be used to compute installation-relative paths, and it can be
any string.

Drop the "Program Files" path completely: it is only usable on English
versions of Windows; therefore, using the NSIS installer to get the
"correct" path to the Program Files folder is recommended, and NSIS
works just as well with any prefix.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>