usb: fix kconfig for usb-xhci-sysbus

usb: fix kconfig for usb-xhci-sysbus

Remove the "default y" for USB_XHCI_SYSBUS because
sysbus devices are not user creatable; boards that use them will
specify them manually with "imply" or "select" clauses.

It would be nice to keep the ability to remove PCIe and USB from microvm,
since thos can be disabled on the command line and therefore should not
be included if QEMU is configured --without-default-devices.  However
it's too late for 5.2 to figure out a place for the DSDT creation code.

Reported-by: Bruce Rogers <brogers@suse.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>