meson: always combine directories with prefix

meson: always combine directories with prefix

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>