When cross-compiling, by default qemu_docdir is 'c:\Program Files\QEMU\'
which is not recognized as being an absolute path, and meson will end up
adding the prefix again.

Add an option to pass docdir location to meson, pre-prefixed like we do
with other directories, build qemu_docdir with the common suffix and use
that instead of config_host['qemu_docdir'].

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200826110419.528931-7-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>