xen/pvh: Use 'depend' instead of 'select'.

The usage of 'select' means it will enable the CONFIG
options without checking their dependencies. That meant
we would inadvertently turn on CONFIG_XEN_PVHM while its
core dependency (CONFIG_PCI) was turned off.

This patch fixes the warnings and compile failures:

warning: (XEN_PVH) selects XEN_PVHVM which has unmet direct
dependencies (HYPERVISOR_GUEST && XEN && PCI && X86_LOCAL_APIC)

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>