uart: Don't change settings or throttle putc for Hyper-V

Azure setup does not like it when FreeBSD overrides the settings of the
UART device. When Hyper-V is detected, don't do this and also don't
throttle putc() output. This is a workaround for the early boot hang
of FreeBSD on Azure.

Tested on Azure, ESXi (VM with serial port), and SG-8200

PR: 264267
Reviewed by: kevans, whu
Tested by: whu
Obtained from: Rubicon Communications, LLC (Netgate)
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC (Netgate)