Meson allows directories such as "bindir" to be relative to the prefix.  Right
now configure is forcing an absolute path, but that is not really necessary:
just make sure all uses of the directory variables are prefixed appropriately.
Do the same also for the options that are custom for QEMU, i.e. docdir and
qemu_firmwarepath.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>