app-emulation/xen-tools: improve ipxe support

app-emulation/xen-tools: improve ipxe support

Add choice to build ipxe from xen-tools or use the system provided
sys-firmware/ipxe package. Since ipxe depends on rombios, make it
and explicit USE. By default, qemu, rombios and ipxe are enabled in
xen so enable then in Gentoo too, with hvm as it's needed for
rombios/ipxe. Dropping unused patches from xen patchset 17.

Closes: https://github.com/gentoo/gentoo/pull/11675
Closes: https://bugs.gentoo.org/682710

Signed-off-by: Tomáš Mózes <hydrapolic@gmail.com>
Signed-off-by: Yixun Lan <dlan@gentoo.org>