x86/hvm: wait for at least one ioreq server to be enabled

x86/hvm: wait for at least one ioreq server to be enabled

In the case where a stub domain is providing emulation for an HVM
guest, there is no interlock in the toolstack to make sure that
the stub domain is up and running before the guest is unpaused.

Prior to the introduction of ioreq servers this was not a problem,
since there was only ever one emulator so ioreqs were simply
created anyway and the vcpu remained blocked until the stub domain
started and picked up the ioreq.

Since ioreq servers allow for multiple emulators for a single guest
it's not possible to know a priori which emulator will handle a
particular ioreq, so emulators must attach to a guest before the
guest runs.

This patch works around the lack of interlock in the toolstack for
stub domains by keeping the domain paused until at least one ioreq
server is created and enabled, which in practice means the stub
domain is indeed up and running.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>